If anything, ARMS regaining prominence would ensure that Spring Man gets promoted in Smash 6, thus maintaining the status quo in this area, in the event that he gets passed over this time.
Spirits are more of a grey area, since it's obvious that things are fast and loose with how they handle those.
Assist Trophies tend to be more of a consolation prize over the fact that they couldn't be considered as part of Smash's playable roster either because
- The character doesn't have enough potential for a moveset
- The character is too obscure
- The character's franchise has been dormant for far too long and the token retro character was already picked
- The character debuted too late
- The timing isn't in the character's favor because another character from that franchise is introduced
It's really no coincidence that the ATs that were promoted are the ones we got.
Punch-Out was pretty much dormant until the Wii game, that's why we got Mac in Smash 4 rather than Brawl.
Isabelle didn't get a chance to shine as a playable character in Smash 4 because New Leaf wasn't even released by the time Smash 4 started being developed and Villager was already a planned newcomer. The Assist Trophy was a consolation prize.
Let's not even kid ourselves. If Echo Fighters wasn't a marketing point for Ultimate, we never would've gotten to see Dark Samus be promoted for Ultimate.
ARMS is in the same kind of situation as Isabelle once was; a game that game out too late to be considered for the base game and given a non-playable role as consolation. They just happen to have a chance to fix that because of the second Fighters Pass.
They don't just promote any Assist Trophy for no reason whatsoever. So not every Assist will have a chance even
if the ARMS rep is Spring Man.
In fact, I'd even argue that ARMS is the only possible case for Ultimate due to the game's timing.