All signs point to assists staying how they are, if Springman and Rex are anything to go off of. Yes there are other reasons that Sakurai may have looked them over, but you’re kidding yourself if you think the existence of the mii costumes was not a factor they considered.
As Someone that does think Assists can be Promoted, here's my Angle
I like to look at the nuance of each assists, because Assists get in for different reasons, and have different Factors from Each other (also Mii Costumes)
In regards to something like Spring Man, considering that Sakurai did say he was Considering Min Min or Ninjara at first, It's very possible the reason Spring Man was added as an Assist was because it was already decided it be between these 2. While ARMs couldn't be considered for Base roster, it's possible they already were considering how they'd be implemented if they did get in, considering that DLC was decided to be a thing in January of 2018.
And I think in regards to New Assists, They could lean more in the direction that they might stay as Assists. Situations do differ between Assists, like for example, if Shovel Knight was negotiated as an Assist early on, but then it's decided they were gonna do DLC later on, I don't think they would throw out the Assist, especially if development is already be started on it.
But Since these New Assist characters were introduced into Ultimate, It wouldn't have been weird if they were not Assists, so making them an Assist could suggest that they were not much on the Radar, and if not much changes in their merit, then it would stay that way. Isaac also falls into this despite being a Brawl Assist, because he wasn't one in Smash 4
Now regarding Old Assist Trophies like Waluigi, Shadow, Lyn, Skull Kid, etc, They have been Assists for awhile and are veteran assists, especially the former 3 as they have been Assists for over a Decade now.
We see that unlike the Playable characters, not every Assist returns, and typically it's because those non-returning assists aren't as popular as the one that do return. Saki, Magnus, and Tingle are examples of characters that didn't return into Ultimate from Smash 4
I Don't think Waluigi, Shadow, and Lyn are on the level of Saki, Magnus, and Tingle. They are above those 3, and it would be weird if their Assists didn't come back because it would be a downgrade in Role. Magnus and Tingle are only Spirits, so by the logic that they aren't Assists like Waluigi and Shadow, they should have a better chance then them in getting into a Playable role in DLC, but that doesn't make any sense if you look at the fact they weren't even popular enough to have their Assists Return.
So if DLC was decided late into the game, Specifically sometime after January 11th of 2018, when the Base game content was set in stone, They made Smash Ultimate without the intention of DLC. If DLC wasn't a thing and Waluigi, Shadow, Lyn, Skull Kid, Ashley, and others got their Assists cut, they would just not have Assists and would have downgraded role
and especially in a case with Shadow where they have to negotiated for the Content with another Company, in this case being Sega, If they negotiated for Shadow as an assist, they aren't just gonna throw that away. that would be a waste.
So regarding Assists and DLC, I think each Assists have different factors that have different amount of Changes to their situation with the Addition of DLC. I'm not sure if Bomberman was negotiated as a Costume at the same time he was negotiated as an Assist. If he wasn't it does show how NPC Roles can be reconsidered.