Improvise action doesn't count because it's unreliable.
Thats only true if you're starting from a presumption of players constantly swapping DMs.
There's not even a basic framework for what it can or cannot do, as it's entirely up to each individual DM
Those are the same things.
For Improvise Action, you literally have to create ad hoc mechanics out of whole cloth, and how effective that is (or is not) is a completely unknown variable from table to table.
Which is irrelevant, as said, unless you're constantly swapping between groups
It
does become an either/or at the players specific table if IA will be a useful mechanic, but that issue is heavily weighted towards the DM being a problem and not merely the mechanic itself.
The game can only go so far to mitigate this issue (and it should), but after a point it has to be recognized that the problem evaporates if your DM says yes, no matter what the game does with IA. A 400 page volume on how to use IA and what all can be done with it is entirely worthless without that basic requirement.
It isn't impossible to have DMs that can run the mechanic with consistency and with an eye towards enabling the power fantasy everyone wants, even without any particular help or guidance from the game. I can personally point at 7 of them, including myself.
But if the game, and the zeitgeist surrounding it, are producing DMs that are so hesitant, if not outright
terrified, to grant even basic things via improv, then there is a much bigger fundamental problem at play than a single mechanic.