You're just going to go on about your hatred for assist trophies until the end of time, aren't you?
Like I get it, it sucks seeing a character you've pulled for get deconfirmed as an Assist Trophy (or potentially a Spirit, but we'll wait on that one). But, at the end of the day, Ultimate had a limited number of newcomers available to work with to begin with and there was always going to be characters who didn't make it in as playable characters. Your issue isn't with the existence of the character as an assist trophy, your issue is that they weren't playable. You would have reacted equally as poorly with no assist trophy of them. You'll be quick to say, "But then they had a chance as DLC" and the honest answer is, as of right now, no they didn't. It was an assist trophy or nothing for some of these characters. The fighter's pass was already chosen ages ago and nothing would have changed regarding their situation outside of being given the false hope that they could still come at a later time. And if DLC ended and they still didn't come, you were going to have the same reaction of disappointment.
At best, removing every single Assist Trophy might get you one character... maybe... depending on how much work actually went into them for this game versus how much a unique newcomer required work wise (which is an absolute ton more work to make sure they are balanced, have the fewest issues of anything in the game, dedicated movesets, etc.). To that end, you'd be trading the following things: Legitimately fun assist trophies such as Sheriff, Flies and Hand, Color Game TV, etc; Isaac's first HD appearance and first Golden Sun acknowledgement since Dark Dawn 8 years ago (Isaac fans were at the very least happy to have that much); the victory of having Shovel Knight as an assist for Yacht Club Games and indie games in general; Fatal Frame; Monster Hunter; etc. You're giving a lot to get maybe one character that you have no guarantee of even liking in the first place because it could be anyone else and we don't know if a single assist trophy would have even been in the running for another spot.
And if you bring up future DLC, well I've got news for you! All bets are off and Assist Trophies could become fighters as could any number of further characters. We're outside of the rules of base game and the first Fighter's Pass, so it doesn't matter anymore who was or wasn't an assist trophy, and I sincerely doubt a potential future DLC character would be held back because Nintendo looked at an Assist Trophy and decided, "You know what, I guess we won't make Isaac fans happy after all, they already have an Assist Trophy" in a hypothetical scenario where they're responding to base game backlash.
Your issue isn't really with Assist Trophies from your posting, it's just with the fact these characters aren't in the game (and this applies to a great deal of people in the same boat) in your preferred role. You've got some legitimate grievances with Assists that are more worth debating, but it really seems like your main vitriol against them comes from this other aspect of things. And you end up using hyperbole to frame it like nobody likes Assist Trophies when in fact many people do, were grateful to end up with an Assist Trophy, and others don't see it as so fatalistic, myself included.