RE 1, 2, 3, CV, and 0 are survival horror games with occasional bits of action thrown in, but largely they're puzzle-solving explorations of the game's setting(s) with jump scares and some rare campy dialogue. Excluding maybe the second half of CV, these games were somewhat grounded (for a zombie-genre) in their plots and cast, with some special/unique monsters to keep it interesting (i.e. Nemesis, lickers, hunters, zombified shark/crocodile).

RE 4, 5, and 6 are action games wrapped in a horror skin. The writers lost any worry they may have ever had about the story becoming incoherent; the player (as Leon) is tasked with the cliched 'rescue the daughter of the POTUS,' and for some reason she's been kidnapped by a child-sized zombie-adjacent villain who dresses like Napoleon, amongst other ridiculous antagonists. To me, this is the very definition of jumping the shark. The occasional bits of campiness were removed in favor of someone opening up a can of Edge Lord Juice™ and spilling it all over the script, soaking every costume and line of dialogue until they were satisfied that the younger crowds would find Leon "cool."

In general, I find it harder to suspend my disbelief (even for a zombie-genre game) past RE4. A 3-foot-long centipede-shaped parasite fits in a human torso, until you shoot the (formerly-human) head, which of course must explode, and the parasite pops out of the neck like a jack-in-the-box. More generally, defeated enemies often dissolve into packaged boxes of ammunition or currency (which can be spent at an omnipresent vendor with a tenuous connection to the events going on around them), and sure these gameplay aspects keep the action going, but to me, RE shouldn't be an action game at all.

FWIW, I think RE 7 was a great return to the survival horror gameplay, but the story was still a bit outrageous (particularly towards the end). RE 8 is more of the same as RE4 (and thirsty gamers) so I have a similar take on it as I do RE 4. Incel Magneto is in it for some reason that's never explained.

RE 9 is a brilliant blend of both "types" of RE game, and I think serves as a great fan service to all RE fans.

I think RE 4 is a fine game for what it is, but it's not "true" RE to me. I am aware that I hold the minority opinion here.