With SAAS, if the software is barely usable but lacks competition, vendor gets paid even if they don't fix bugs or broken functionality. When paying up-front, there always is competition - your own old version; so the vendor has strong financial motivation to make improvements since the recurring "maintenance" upgrade revenue is conditional on them, unlike in SAAS.

The difference is that with upfront payment developers are forced to actually add features that provide more utility. Otherwise customers don’t upgrade. With SAAS you have to keep paying, even with if the software is completely static with no new features or bug fixes.

As for bug fixes, do you think I am more or less likely to recommend your software to my friends if it is full of bugs and you don’t fix them?

In the case of Sage, the difference was about 500% cost increase for each of my two small businesses.

You buy what is offered (and a support period in most cases). Not a promise. No one buys a consumer car and expects it to run on water the next month.