Redglyph
proud GASP member
They can and do all that now, and charge for all of it piecemeal. If anyone complains about the quality of what they're playing or the 5 fps, they can just say "it's in development."
But I think they could keep doing that even if they released it officially. Such models already exist, Eagle Dynamics do that with DCS simulator, or Laminar Research with X-Plane. It's in constant evolution even though it's released.
With DCS there are stable and beta branches, new aircraft modules are released now and then (not everyday, those are quite complex add-ons), with an Early Access phase, there are periods where campaigns or modules are broken (even on the stable branch). Of course people are not always happy, but they stay because they know it'll improve, and overall it's been working pretty well for years.
No microtransaction, virtual money and so on, of course, they're honest sims.