Why is this an argument? The games you mentioned were limited by their hardware of their time, and ran and looked beautiful back then as they maximized what they had at their disposal.
Sword and Shield genuinely runs at 15fps at certain points and re-uses 3DS animations. Textures are missing, and they have cutscenes that look like early 2000s stop-motion animation. It looks like a GameCube game in these areas.
Graphics have context. The visual style is key. Sword and Shield do not have a pleasing style in areas like the wild area and battle backgrounds, nor is it consistent. Undertale has a style that fits and is consistent throughout the game.
The difference is that they said they removed content to improve the animations and graphics. They did not do that.
I think there are two different arguments to be had here. The first is that "gameplay is more important that graphics", which, I believe goes without saying. Certainly, games have context to why they have the graphics they do, but it's still flawed to say, "If it looks bad, it sucks." Graphics being "bad" is subjective anyway, but there is nuance in the art style/graphic style chosen for specific games.
When it comes to Sword & Shield, I'm not making excuses for Game Freak. I was making the point that I will likely enjoy the games because of the gameplay that Pokemon offers, and that that is what makes it consistently a best-seller. Graphics have never been the selling point. The gameplay is what sells it.
Granted, the flaws being that many gameplay aspects have been cut, the animations aren't what they said they were going to be, and they
should be able to do better. I suppose what one could say is, that Sword and Shield won't necessarily be bad games, but bad
Pokemon games. At least, that's more or less what I'm expecting.
The weird thing in all of this is that while it may seem as though I'm disagreeing with people who are upset, I'm really not. I'm mainly just arguing against negativity. We could sit here for days talking about how bad things look, but despite these things, I'm sure I'll still have fun with the games. To me, that matters most.