Sorry for the Reddit-spacing, I think this would've been a gross wall of text otherwise.
There is no unfathomably awful mainline Pokemon game. Some are better than others, but there is fun to be found in each one despite any flaws. People here over-exaggerate issues.
Gen 5's animated sprites were fun.
Vanilluxe is great.
Lillie didn't need to exist. Cosmog escapes by itself, takes a liking to Hau, and Gladion takes charge in the plot against Lusamine instead.
Pokemon Sword and Shield looking like a sixth or seventh-gen game is a valid complaint. Graphics don't make the game, but it's one of the things people are allowed to look forward to- especially when the leap is from the shoddy 3DS to the much more capable home console/hybrid Switch. Gamefreak has no excuses for their shoddy work anymore.
People have the right to be pessimistic. Gamefreak has been doing the series dirty since XY, only natural that some people would be disappointed/have their hopes low.
"It just gets bigger" isn't always a bad thing. Sometimes the Pokemon changes too much for its own good by losing the theme that made the Pokemon enjoyable in the first place.
Diantha has a great design, especially as a champion. Gamefreak just fucked up big time with the game's general difficulty, her team, and not giving her more appearances.
IVs and Natures are not fun game mechanics and were flawed from the start. They make sense from a world-building perspective, but one shouldn't forgo playability for that.
EVs should have a slider like what you see on Pokemon Showdown. You'd still have to earn them before you can apply them, though.
RSE's music and sprites were lackluster.
ORAS's redesigns for Team Magma and Aqua were great. Really pushed the "brains vs. brawn" theme.
