One of my first posts on the forum and on a controversial subject - please bear with me
Upon hearing the news that there would be civ-switching in Civ 7, my first reaction was that of shock and horror. Being a massive Civ fan (since Civ1) I did end up buying the game, though.
I think there are (at least) two kinds of Civ players: 1) those approaching it as any other game and being interested in min-maxing and such, and 2) those looking for historical roleplay. I don't think the designers who have made this design choice fully appreciated how much cognitive dissonance ahistorical transitions would cause for the latter group. Some of my favorite childhood memories were of desperately defending the Romans from invasions (I must have been a pretty weak player, so these did happen) and succeeding. This design takes that away from me.
The real issue too might be that the game is basically the same since Civ 5 and this is essentially the third time the same paradigm is used. We still have the same hexagon-tiled cylindrical map, cultural and religious gameplay etc. that we had all the way back to Civ5, and there's only so much innovation you can get through tweaking the ruleset alone, which might have prompted such a drastic change. I've recently completed a playthrough of Civ 5 and 6 each, *after* buying Civ 7. I was amazed at how much even the graphics still felt fresh and in case of Civ 6, even possibly better than 7 in some ways.
So, I dislike Civ transitions, but it is what it is. I actually don't want them to do a Classic mode, now - I don't think that would result in a good game. I'd rather Firaxis focus on giving us more Civs for historical transitions - I can only really play that way, and right now there's a limited number of viable paths. Even doing some sort of a Civ 6 remaster (like Test of Time for Civ 2) would IMHO be better than a classic mode, for supporters and detractors of civ switching alike.
As for Civ 7, I think the best way to proceed now is to give it a reasonable, but shortened, lifespan - like what happened with Windows 8 or the Wii U. After that, we could have a proper, non-rushed, Civ 8 for the late 2020s and beyond, with a different paradigm than the "Panzer General" one of 5, 6 and 7. I have some thoughts on what that could look like, I hope to put it in a post one of these days.