There lies your basic error. If people disliked change, why would Civs 2 to 6 have been successful with all the changes they introduced? If you were right no gaming series could be successful, still here we are at iteration 7. The question is do you change things that improve the game for your target group or do you worsen things? The overall reception of Civ 7 by the players indicates the latter.
Yeah, I really hate the subtext of this post. The fact of the matter is that Ed Beach is pretty clearly obsessed with Amplitude games (don't be surprised if we hear he's stepping down and shortly after becomes a lead for whatever game there now that he's pretty clearly not going to be Civ VIII lead). His big mark on the series has been making the games more and more resemble those games. Some of these "Amplitude signatures" are well received, notably asymmetric civilizations, building on the map rather than the city, and caring more about a neat idea than game balance, but the bottom line is that they're second fiddle for a reason. Civ VII copied the homework of their widely panned Humankind heavily. The game aesthetics are nearly identical, the era system with victory points is very similar (albeit not exactly rocket science to come up with), civ switching is nearly identical, the expansion limiters are nearly identical, the town system is heavily inspired by humankind, and ironically, they even copied the lack of asymmetry in Humankind even though that's kind of Amplitude's thing and every 4X ever that successfully pulls it off, including Civ VI, gets praised for it. Nobody should be surprised that running back these disliked mechanicsended up with a disliked game.
I'd also argue that Civ VII failing commercially is the absolute best thing that could happen to the genre. Between Amplitude and Ed Beach being Civ lead, "4X" has largely meant "games where you build on the map with a design philosophy of the more systems you have, the more fun your game is," and the genre never meant that prior to ~2015. The writing has been on the wall for a while now that the genre was on pace to be like action games where I hope you like Dark Souls because everybody is just making their take on Dark Souls. Things are just better when you have Paradox making Paradox games, Amplitude making Amplitude games, various AA devs doing what they want, and Firaxis making pick up and play games. Amplitude making Amplitude games is plenty, and just because I've seen calls for it on other forums, Paradox making Paradox games is also plenty. Firaxis doesn't necessarily need to stay in their particular niche, but it should happen because they want to do something different. Not because the developers wish they worked for Amplitude but Firaxis pays better.
Is there a different better example where fan hate rather than a low quality product ended a popular franchise permanently, and where nobody else took up the mantle? Just that franchise is dead forever, killed by its ignorant fans who couldn’t appreciate something amazing?
Deus Ex is probably the closest, but that's more people understandably boycotted a game that ends halfway through the story, has consumable single player DLC, had large portions of the game act as ads for their mobile game, and very unrealistic publisher revenue expectations. In general, this doesn't really happen for obvious reasons.
2. All those posts assume Civ7 is a commercial failure. We actually have zero information on it. Unlike HoMM4, which had changes for the sake of changes (or fan requests), Civ7 changes are aimed at grabbing new audience, so mixed reviews from old fans (as you correctly pointed out negative reviews talk about changes, so they are from old fans) don't speak anything about the game will be accepted by new people. And mixed reviews aren't negative, they are 50/50 and usually they don't prevent people from buying a game. We'll have much more information in about a year, after summer and autumn sales will pass and 2K will publish financial results incorporating them.
We don't have a smoking gun I guess, but the radio silence on sales numbers besides preorders is deafening and the concurrent player numbers are horrendous given the game budget and popularity of the series. Arguments against steam playercounts for an established game released in this era are more or less Russell's teapot. Sure, it's not impossible that steam is an outlier, but it'd be awfully weird if it was. Nothing Firaxis did should have made players run from steam. It's not like FFXIV where you're locked out of third party promotions if you buy from Steam or a game where its popularity predates the platform. There's a lot to indicate a commercial failure and basically nothing to indicate a success.
is „shallow“… i would rather say the criticism is justified. 
No need to repeat it again and again, but it‘s our right to criticize the game if we don‘t like it. Shallow or not shallow, it remains justified.
