Eh, I'm gonna have to disagree with you, for a few reasons.
Sadly, I think Ashley’s chances of becoming playable are much worse for Smash 6 than they were for Smash Ultimate. I was very optimistic about her chances this time (nobody is a shoo-in anymore, but I thought she was one of the most likely contenders and very underrated by the conventional wisdom). But it’s quite likely she becomes less popular and less relevant by then. We also don’t know when or even if Smash 6 will come out.
Why is it likely that she becomes less popular? Despite two relatively lackluster releases between Brawl and Smash 4, Nintendo of Japan suddenly deemed her a marketable character around 2015. While we on this board chalked that up to a good thing, we didn't take into consideration how that wave of marketability may have come a little too late, relative to Sakurai's development plans. Even now, if you read through some of the blurbs in WarioWare Gold, Ashley is obviously still a popular character. It's just a matter of seeing what Nintendo decides to do with her next.
As far as Smash Ultimate goes, the newcomers are all in one of three categories:
1) Popular picks from throughout the years - these are your Ridleys, your K. Rools, your Simon Belmonts, stuff like that. The characters who have been at the epicenter of Smash speculation for at least a decade or two. There aren't too many of these simply because of the time crunch.
2) Relatively popular characters who would make for great Echoes - Daisy, Chrom, Dark Samus, (probably) Shadow, (probably) Isabelle, (probably) Dixie Kong, etc. They take minimal development time, and make a lot of people happy. It's a win/win.
3) Inkling - Despite being a new character, their franchise quickly became one of Nintendo's core IPs. You don't make a new Smash and not put in the Inklings. (It doesn't hurt that they were probably fresh [no pun intended] on everyone's mind during the Ballot).
This is what I attribute to her not getting in: despite her increasing marketability, her popularity (up until mid-June, evidently) wasn't as incredible as King K. Rool's, and she's too unique to be an Echo like Chrom. While a normal Smash development cycle would probably grant her more favor, a cycle where the emphasis has been on bringing every veteran back plus a few titanic newcomers was just too competitive. But that shouldn't speak ill of Smash 6, or even Ultimate DLC.
As a beside, there's no way Smash 6 isn't happening. Even if Sakurai decides to retire as Smash Director, Nintendo will make sure that he bestows his blessing upon a new director. Remember, Smash is a AAA console-selling titan of a franchise. It's not an artisan indie graphic novel that can't make a sequel because the creator doesn't want to compromise aspects of the story, it's a series that Nintendo, as a business, wants to keep afloat due to the money it pulls in.
Unfortunately, this timeline looks more realistic:
1. Ashley is not playable in Smash Ultimate.
2. Warioware Gold sells poorly largely due to being on an essentially obsolete console and a lack of good marketing.
3. Warioware goes the way of F Zero and Ashley is forever confined to obscurity.
I don't see the logical correlation between Step 1 and Step 2. WarioWare Gold, despite being out for less than a month on a dying console, seems to be doing alright. The series itself, despite being niche, seems to have a few "million-plus" installments under its belt, and Gold is a return to form, plus some. Not to mention, outside sources such as YouTubers and game journalists seem to be eating the game up. And if you're only concerned about marketing in so far as having a character in Smash, then don't worry: Wario is still a playable character.
Also, I wouldn't use F-Zero as an example, considering it is the complete inverse of what you are predicting for WW; where you seem to think WarioWare will sink due to a lack of a second representative, F-Zero sank IN SPITE of having a playable rep in every Smash game.
In this case, we need two things to happen:
1. Smash Ultimate needs to have DLC. This is very likely, assuming Nintendo likes money. Splatoon 2 keeps getting DLC, and Nintendo will need people to stay interested in its multiplayer games if their subscription service is to have any chance of success. A steady stream of new content is essential for doing this.
2. Assist Trophies need to be eligible for DLC. Pretty much any sign here will do. Sakurai just needs to say something along the lines of “anything’s possible”, “everyone is eligible”, etc. Or better yet actually reveal an Assist Trophy as a DLC fighter, even if it’s someone stupid (not naming any names).
If we grant 1 and 2 for the sake of the argument, Ashley’s chances suddenly look pretty good. Smash 4 had 7 fighters developed after the game came out. Three of them were veterans. Also, Smash Ultimate is coming out earlier in the Switch’s lifespan (which will likely be longer than the Wii U’s) meaning that the time until Smash 6 development starts (if there is a Smash 6 and if the Switch does get a successor - two open questions) is longer. That means there might be even more DLC, perhaps at least ten newcomers. Even more if they decide to keep expanding Smash Ultimate indefinitely rather than making a new Smash.
So if everything lines up, Ashley would need to be among the top ten most wanted newcomers (including assist trophies that have already been revealed, but hopefully excluding non-video game characters like Goku, Shrek, and Jeb Bush’s dad) to have a good chance at becoming DLC. I think she has a good shot of getting there. The more fighters that get revealed and removed from contention, the better.
Another possibility: a character with a similar size, shape, and move set to Ashley is revealed and she becomes an echo fighter.
That's what we're rallying behind: playable Ashley DLC in spite of her Assist Trophy. Sakurai doesn't need to say anything because, as was mentioned above, people will come out of the woodwork to give Ashley a run for her money. As it stands, a lot of other AT fandoms appear to be accepting defeat, where Ashley fans have taken to continuing support in hopes of getting DLC. Sakurai obviously knows about Ashley's growing popularity, which is presumably why he pulled the plug during the Smash Direct and showed her as an AT. This is a good first step for us. We just need to keep it up.
And I agree about the DLC thing. Not only is Smash coming out early on in the the Switch's lifespan, but it will inevitably sell better than Smash 4, on a console that has already outsold the Wii U. Without a doubt, if DLC happens, it will be more expansive.
I still think you're downplaying Ashley's odds, though. Sure, there are the goofballs who suggest Shrek, but as far as Nintendo characters go, who else is left, as far as actual marketable/popular/unique characters go? I would argue that pretty much any super Nintendo All-Star is already in the game, so now is the perfect opportunity for those smaller-scale superstars (like Ashley) to shine, assuming Sakurai is willing to promote Assist Trophies. Assuming the fan-rule of "Assist Trophies can't be playable" isn't actually a thing, and third-parties aren't as involved as they were during Smash 4's DLC, I think Ashley automatically floats to the top. She's popular, she has the capacity for a unique moveset, and she has an interesting personality that would also meld well with the rest of the cast. That is literally everything Sakurai looks for in a character.
As far as an Echo goes, I'd be down, because I still get my Ashley amiibo and whatnot, but I'll be darned if I can think of a character she'd be an Echo of.
Glad to see you're back,
Jazzy Jinx
! We all seem to be in good spirits, working to maintain support and hoping for a DLC reveal. Even if that doesn't happen, at least in my opinion, Ashley isn't a matter of "if", but of "when".