Maybe it's because I'm seeing it from code/modder perspective.Civ Switching isn’t just cosmetic though. You lose access to most of your previous abilities every time you swap. When I go from Carthage to Spain/Abbasid/whatever I’m not just changing my name and banner, but also my entire game plan. Even if my civ was still named the same, the core of what made Carthage interesting and different is gone.
It's the age transition that change the rules and your game plan.
As in: the entire game's database is actually wiped and a new one is created at each transition, and you're playing a different game after reloading.
Even if you would keep the appearance and abilities, you'd face a new set of rules.
cosmetic is easy to fix is what I mean.I have issues with both. If you think things like cosmetics don’t matter, just recall all the sturm und drang surrounding Civ6’s art atyle
keep the name, appearance, no problem.
keeping the abilities, more work because of the database wipe, but still possible. Its done already (see Enduring Empire)
and now you're left to deal with the age transition.
here makeup won't help you, its the core of the game, you may be able to cancel some rules change, but I doubt we'll see a version of civ7 without a loading screen between ages or re-adding one of most the important element of a civ game like in <insert which civ version was the latest true civ game here>:

(ok maybe the later is possible with civ-specific tech trees, but you'll never have civs in different eras)
and also:
this, completely.This speaks to the Brontosaurus stomping on the Game Design: Ages.
Whether they call them Eras or Ages or Artificial Nooses to Strangle Game Play they've been with us for several renditions of Civ, and have been picked up by other 4x historicalish games like Millenia and Humankind so that they appear to be a Basic Requirement to such games.
Which they aren't. They started as an historiographical 'shorthand' to keep track of changes that defined certain periods in certain areas of the globe but, critically, no one has ever agreed on which ones apply for which times to which areas and no one has ever seriously suggested that they apply to all areas of the globe simultaneously unless you so load them down with caveats and exceptions that they become (to mis-quote the old physics joke) like Spherical Civs in a Vacuum - worthless for any real purpose.
Full Disclosure: I've never liked them, no matter how convenient they may appear to the designers to grind the game forward defining what you are supposed to be doing at each stage.
Civ VII has not improved the design one whit over previous attempts, except to reduce them in number - which is counterbalanced by making them absolutely definitive in what you are allowed to do and still almost entirely Eurocentric in their definitions and effects.
So, while we are all discussing 'Civ Switching' or 'Legacy Paths' let's not forget that both are tied, twisted and entangled in the Age system that really defines the game and every mechanic in it: remove Civ Switching and you are still throttled by an artificial construct that severely and arbitrarily limits everything you do.