And people say they would've been happy with an updated Civ 4 with better graphics, etc. I'm long bored of Civ 4, 5, 6, etc. by the time a new one comes out. The series has been successful by evolving, even if 7 is more of a miss so far. If it was just same game, better graphics, it would've died years ago.
I'm not even sure that it's that, as much as they made some poor decisions/execution with the changes they made:
1) Bugginess/UI issues - definitely a turn off for people. I've had Civ 7 crash more in the under 100 hours I've played it than in thousands of hours of 4/5/6. This will eventually get fixed, but it definitely leave people with an unfinished feeling.
2) Civ switching - if they were making this choice, imho, they should've been a little more consistent with the initial leader and civ choices, so there was a little more throughline (and the AI followed that throughline) and gone for more vareity later on. They did it with India and China, but also more like Maya -> Aztecs -> Mexico etc. This should also eventually get better with more civs released.
3) Ages - the choice to make it feel like a reset instead of a progression, seems like potentially the biggest miss, and the hardest to overcome. They've consistently said a lot of players don't like having their civs regress (ie Sid claiming early on they wanted to put dark ages in Civ 2 or 3 iirc, but had to drop it based on testing since players just reloaded). Maybe they should've done a bit more gatekeeping - ie Ancient era you can only expand to 2 rings. Exploration 3 rings. Modern era you can have 5 ring megalopolises, etc. Maybe even save the Town/City dynamic for the exploration era - everything builds in the ancient era, exploration era you can chose to have towns feeding food/production to 3 ring cities. 5 ring megalopolisis can incoporate smaller towns. Etc.