If the 'frontend' stays the same (and i literally mean all frontend, from GUI to APIs), then noone cares.
If something changes, you atleast can delay the change, until you fix other components (or maybe even not update, if it's some internal stuff), because with SaaS, you have no say about it.
Even if the project becomes unsupported, you can still run the old version, until you find 'something new' (or again, keep runing the old for internal stuff).
If you build anything around a SaaS provider, they alway have yu by the balls,... be it with ever changing pricing or killing the project, "just because" [0]
The moment that a major security bug is discovered in your app, you will need to upgrade it. It's not as simple as just saying that you can still run the old version. That's just one issue of many. There are many, many more hidden costs than you're giving free software credit for.
Of course... if it's outside facing... internal stuff can be delayed (and often is).... but again, that's why you pay admins for. Usually feature updates (api changes) and security updates are kept separate (unless you waited with the feature update, until there was a security issue).... but again, the thing is still yours.
it's like a taxi vs a free car + free parts, and all you need is gas and time to do service.
Yes, that's the exact point I'm making. You have to pay someone to manage something like this. The point (edit: okay, not the point) of this sort of software is to try to minimize those costs, but at the end of the day you still have to pay for them, and often even more.
You still need an admin, even if you use SaaS... someone has to make the SaaS work... this just adds an aditional update every now and then to their workload.
Clearly you've never had to go to hours and hours of change management meetings for a minor release.