The mechanisms for paying for major additional software development through the MAS are clearly not yet resolved - hence I would guess, the difficulty for Omni in offering a simple response here.

Economically, however, the necessity of paying additional sums for major upgrades is effectively inescapable (short of a halt to further development).

Underpriced tickets to an unlimited stream of future values, financed only by the need to make continuous new sales to late-coming aspirants to the status of "life members" ... well, this we have seen before ... It is essentially a variant of the Ponzi/Madoff model, and it never lasts long ...

(The market for new takers saturates, and then things begin to unravel).
This is not, I think, the course that things will take ...

At the moment the iTunes/MAS model is that any major new development has to be a new product which you simply have to purchase anew without any upgrade discount at all. People have been willing to put up with this for the small $5 values, but there will clearly be resistance with products that are in a significantly higher price range ...

I think it is unlikely that the MAS will turn out to be a cheap ticket to the land of cockaigne ...

The greater danger, at present, is that it lacks the mechanism for giving you any discount whatsoever when you want to buy the next version ...