A few people touched on it here before, but I think there's still a reflection to be had within the argument of how "upgrades" of Assist Trophies and Spirits would work. I've dialed back my skepticism a bit around the topic, but I'm still of the idea that it's not a "one size fits all" concept, if you will. It's not of the mindset to say, "Wow, you want THAT character?" but more along the lines of, "What are the individual likelihoods of individual characters getting upgraded?" I think that's the better question, and it doesn't necessarily require personal preference.
---
When it comes to Spring Man, there's a few things here to consider.
1. When it comes to ARMS, there's no "main character" but based on advertisements and evidently the way Nintendo looks at the characters, Spring Man and Ribbon Girl are typically at the forefront.
2. No series in Smash has ever had a non-mascot of a particular game/series as their first character to join Smash
3. First party newcomer DLC (except for
, who is an exception based on originally being planned for base roster) has been promotional in nature and has, as have all series with playable characters in Smash, stuck with the protagonists or mascots for their particular games.
4. I would argue that being an NPC isn't as strong of an argument anymore considering Chrom, this entire time, is a playable character AND final Smash. Meaning, you can summon Chrom to beat up Chrom, and then after you win, can have Chrom clap for Robin and Chrom in the victory screen. Now obviously, this is an exception, but
could easily apply to ARMS as well when in tandem with point #2. There's also the fact that certain AT's are turned off for certain stages, meaning Spring Man could easily be switched off when playable Spring Man is on the field...or not.
---
Now, this isn't to say that "upgrades" can't happen, but I think it has more to do with context. Spring Man as an Assist Trophy upgrade isn't the same as Waluigi as an Assist Trophy upgrade. ARMS is an IP with little content in Smash overall, and it can bring plenty that is new to the game, including a playable mascot in a mascot fighter. Waluigi is not the main character/mascot of his home series, nor is he from a series that could use a boost in the publicity generator that is Super Smash Bros. It's a very different situation.
It doesn't mean Waluigi can't get an upgrade to playable as well...it just means that Waluigi would need an
alternate path to becoming an upgrade. Could popularity be it? Perhaps, however I personally don't think popularity is some "be all end all" quality that gets folks into Smash. Isaac is from a series with no playable content in Smash, but Golden Sun is a dormant series. Adding Isaac as an "upgrade" wouldn't work as well with upcoming releases or IP that Nintendo is pushing, which is what Point #3 suggests, with us only getting Byleth and Corrin as DLC newcomers from Nintendo.
Now, it's not to say that a character that doesn't fit those criteria couldn't be DLC. It
could happen.
But based on what we can refer to, why would it? I mean no disrespect, and I am completely open to conversation about certain picks and why they specifically could be DLC picks. But frankly, "moveset potential" and "Just because it hasn't happened doesn't mean it won't ever happen" aren't exactly amazingly strong arguments in my humble opinion. (I'd say popularity as well, but popularity can matter in tandem with other factors imho.) Those are points that could be in favor of any character and therefore don't work as an advantage to any particular character, at least from my perspective.
---
Anyway, I think the biggest factor with upgrades will depend upon how a Spring Man "upgrade" is discussed by Sakurai or a speaker in a presentation. If they don't make a big deal out of that? Maybe it won't mean anything going forward. If Sakurai says something that skirts the line of "Was an assist trophy, but I guess you could say this is a special case"? We might have to consider that as a blow to upgrades.
I don't think he'd ever outright say, "Yes, it's happening more often!" or, "Nope, sorry!", but I think inferences are key here.