Exactly, playing without ages and switching doesn't work as a side mode (or even main mode). It requires a redesign of the game and than this would be the only option to play it. Which I think is unlikely. Having ages and classic mode simultaneously in the game doubles the workload: half of the leaders would have different abilities, in one mode buildings and great works go obsolete but are permanent in the other, in one mode yields on tiles are revamped but not in the other, in one mode legacy points can be used to grant additional bonuses but no in the other, etc. It's impossible to keep both up and 'balanced'.
They already made it years ago, and I don't see any indication that they've changed that. Ages are in, and stay in. All that's happening is smoothing out the roughest edges so that players that were put off by resets, crises, etc aren't as appalled.