I refuse to talk about things in terms of "can or cant be done". When i talk about Classic Mode, i talk about what i think NEEDS to be done
If you asked me 5 years ago if rockets can be recovered after launch i would have told you that its impossible
To me, Classic Mode need to be free of interruptions, it needs to be a smooth fluid sandbox experience. Its not my job to tell Firaxis how they need to do technically.
I work in Development and "that cant be done" often means it would take more time than its worth. I think even if it takes them months to change the Engine to allow that, it would be time well expended
it just took 70 years from Tintin to SpaceX

can't be done for civ7 means exactly what you mean: more dev time than it's worth, if things are at this point, better start from scratch and call the result civ8.
people here hope for a game option for civ7, one available in one of the expansion, not a rushed civ8 in 5-6 years (at best) while civ7 will be left dying by lack of new content.
I'm not sure about this. I think the Modern Age is perfectly capable of bearing the asset load of all three ages.
just google civ6 asset limit, and I've not found any evidence that they changed their game's engine.
Just put all the techs etc in Antiquity.
then code a new UI to handle them. copy all units. buildings. modifiers. well basically merge the 3 databases, referencing all assets.
and then try to launch the result.
if it works (at least on a gaming PC), congrats, that's one of the case where I'd be very happy to be proven wrong, would give me some hope that I could have fun playing/modding civ7 at some point.
ho, you'll need to rebalance everything after that.