The issue comes with treating an assumption as a rule. Because that's all the "Assist=No DLC" line of logic is, an assumption at best, and a self-fulfilling prophecy at worst. If people go in assuming it can't be done, the demand dies down and then it won't be done.
That's why it's better to assume that no option is truly off the table. If it doesn't happen and some folks get disappointed, that's on them. People shouldn't feel the need to police others' hype anyway.
I'm just trying to be realistic based on precedence and how the game is designed. If that makes me police the hype, so be it because I'm not the one creating conspiracy theories, grasping at straws and trying to twist everything into "the thing I want has a chance!!!" only to be disappointed when the final product comes out.
The truth is that even if returning Assist Trophies are ports from previous game, it doesn't mean there isn't work put into them for the new game.
You don't just port stuff by copying and pasting the assets into the new game, especially when the new game has an updated engine and everything else needs to be reworked to function properly within the engine. Sakurai noted exactly about this when discussing the returning characters and how even characters from the previous game (and some obviously look ported from the Wii U game) had to be readjusted to fit with the new engine.
It's not as straightforward. The thing with promoting Assist Trophies into playable characters through DLC would mean that the game suddenly has a bunch of unused data lying about there that wastes resources when loading the game card and all that. It becomes data that has to be loaded, but is completely unused. That is bad programming and design by default.
When Sakurai considered DLC for Smash 3DS/Wii U, he wanted to focus on completely new content that wasn't in the game at all. Missing veterans to appease fans and completely new characters that had no presence in Smash before, including 3 third party franchises that got newly represented with unique characters that stood out from the roster.
Regardless of how the final roster turns out for Ultimate, if DLC is to come, I expect completely new content, and a once again high focus on bringing new third party franchises to the game, highly requested characters not found in the game or Nintendo releases that missed the base game's development.
Look, if I'm wrong about this, I'll be wrong about it and it's a situation that, for me, would be unexpected and it would create a precedent. I just think it's unrealistic to expect simply upgrading an Assist Trophy to a playable character via DLC when there are tons of other potential offerings that could be DLC and completely add to the game.
Because otherwise, you're not adding to the game. You're removing a feature from the base game as a cost to add a new character, which is a fairly odd premise for DLC in the first place in a design standpoint.