> And what does the customer do if the vendor has discontinued it? Or charges for an upgrade? Or has gone out of business?

Those can all be filed under Not My Problem (as in, Microsoft's problem,) and safely ignored. On the other hand, when Highly Influential So-And-So upgrades from 3.1 to 95 or whatever, and Very Population Application v4.9.6 starts falling over, Microsoft gets the black eye whether they deserve it or not. The whole equation changes.