I wholeheartedly agree with everyone on this.
I can think of a few reasons why Ashley's support has been hit harder than many of the other disconfirmed characters. Some of that support was unfortunately only due to her being a bandwagon choice between E3 and the August direct, and that bubble burst immediately after her AT was shown. Others were put off by some infamous comments made by a couple of Ashley fans out there. WarioWare Gold also helped the hype, but that's old news by this point.
But I think there's still plenty of support to be had. There just needs to be a greater sense of direction beside just a Smash appearance in order to get people engaged about Ashley again.
Look at some of the other characters whose threads are still active. Banjo? Geno? Isaac? These are vocal communities because the they need to be active. The stakes are higher for them than just Smash. If their support isn't heard, these characters may never make a meaningful appearance again.
Conversely, there is no major reason for concern that WarioWare or Ashley won't be back. The only question mark was Smash Bros, and once a playable appearance obviously wasn't going to happen, there wasn't much to talk about other than patiently waiting for the next Wario game.
It's a good thing that we don't have to be loud just so our character will make any kind of appearance, but it is obviously problematic in getting much traction for anything beyond the expected WarioWare role. So we need something more. Unlike asking for a new Banjo or Golden Sun game, campaigning for a new WarioWare is kind of redundant, so asking for these characters in spin-off games is the next logical choice. And really, why shouldn't they be? If we can make our interest known, maybe we could get Ashley instead of Patina Copper Baby Rosalina as our next Mario Kart character. And the more places she appears, all the better for Smash as well.
This might be something that could be done in cooperation with other characters from series that are loosely tied to the Mario universe (WarioWare, Wario Land, Yoshi, Donkey Kong, etc.). It does have the potential ill effect of making Ashley get lost in the shuffle, but having a louder voice might be worth the risk. If Nintendo did dip into these franchises more, I trust Ashley wouldn't be too far down the list of options. They've already shown her special attention before.
I've noticed a few things about Ashley and the level of attention she gets, so prepare for some observational rambling:
1) You are right about the E3 bandwagoning. Heck, it's what led me to come back to Smashboards, the difference being that I actually stuck around because I legitimately want Ashley in Smash.
2) Smash speculation heavy-hitters seem to revolve heavily around nostalgia, and Ashley is a relatively new character, especially compared to characters originating from the SNES and N64. Like you said, characters like Geno and Isaac are faced with an extreme situation of either maintaining a constant Smashboards presence or being obsolete, but they are also characters that their respective fans grew up with. And since SNES/N64/GBA fans are generally older than DS fans, they probably have more of an influence due to their seniority, at least on SB.
3) WarioWare has stumbled quite a bit. Snapped, G&W, and even WarioWare Gold, to an extent, are far cries from the success that Touched and Smooth Moves saw. Gold had the potential to be a return to form, but it was at the wrong place at the wrong time, whereas Game & Wario was simply underwhelming...and on the Wii U. This increase in obscurity, while not a threat to the fate of the series, has possibly shifted the franchise to the background, covered up by up-and-comers like Xenoblade and Splatoon, as well as the nostalgic picks mentioned above.
In short, Ashley was the bandwagon pick that people expected, rather than actually wanting her. Now that she's been disconfirmed, actual fans have scrambled to characters who have some degree of hope, while Ashley herself shifts to the background due to the nature of her niche franchise, making way for fresh faces with mainstream appeal and nostalgic picks who will be ever-present until they miraculously make the cut. That's my take on why Ashley's support has seemingly tanked/gone quiet, and I'd say it lines up nicely with your thoughts.
Anyhow, enough of that. As for the Mario Kart thing, I think the only other Mario spin-off characters with more of a chance of appearing are Dixie, Funky, Poochy, and, to a lesser extent, K. Rool. The former three all have better mainstream appeal than Ashley (Dixie is one of the three main protagonists of the original DKC trilogy and a fan-favorite; Funky is a nostalgic character given new life via Tropical Freeze's Funky Mode; Good-Feel has been pushing Poochy as a secondary mascot to the Yoshi series, what with the amiibo, Woolly World port featuring his name in the title, and appearance on Crafted World's box art (I think)), and K. Rool...well, I'm sure Nintendo took note of just how marketable K. Rool is. But once those four are in, Ashley suddenly floats to the top, especially compared to her WW colleagues, who don't have the same tendency to appear in crossovers.