I guess it all depends how much you can imagine it can be 'ironed out'. The age transition is such an enormous part of the game that will always be hard to ignore.
I can imagine a world where all 3 mini games feel fulfilling and viable, and give you enough direction whilst keeping the continuity with what happened in the previous age. I'm not totally against the age transition per se. I think they need to deal with the enormous sense of loss that can occur in that transition. However I think there is a conflict in the vision of 3 separate games, with the idea that users could start in any age and one of continuity. It will be very hard to satisfy both visions.
I don't know that this is a dichotomy you can fix. Part of the goal of distinct ages is that play patterns be very different. Even if all 3 ages were perfectly balanced, players would still inevitably have favourites. It's part and parcel of what they were trying to achieve.
As it is though, antiquity seems to have almost unanimously picked up all the best versions of the legacy paths. Which is kind of the worst place for firaxis to be in.
I would be excited to start a new age if it didn't feel like I have to start all over again. Too much effort was put into pulling players back and rubber banding. In reality they should have made the ages more distinct, so that the advantages gained in one age don't always translate exactly into the next. If an age is suddenly about exploration and colonisation, then having a huge land empire could be more of a hinderance for example. I'm not sure they have approached it in the right way.
I think a large part of the issue is that once you start playing as you describe - building for the next age not the current one - the rubber banding doesn't even work... This might be the Civ version with the worst snowballing effects once you adjust your play style.
But it also creates a delayed gratifcation effect, where you are building to be better later. And the payoff isn't great right now.
I was initially pretty bullish that it was a good idea to try the changes Civ7 made. They were trying to address longstanding issues with the game. Unfortunately, the devil's in the details and I think they've failed to achieve their goals (snowballing still there, no incentive to play to the late game) while creating a bunch of new own-goals (jarring transitions, variability of how fun the ages are, a general revolt against civ switching)...
I really hope the first expansion is effectively a backpedal on Civ7's 1/3 new concepts. I don't think the game will survive otherwise, and that would be a real shame because the 1/3 iterative improvements Civ7 made (no builders, army commanders, influence, etc...) are beyond fantastic and are the perfect foundation for rhe best Civ game you could make.