Sure, I could build a photo app for a one-time purchase that lets you add your own credentials for S3, B2, etc.
What’s the market size for users who will set this up themselves (versus going for a competing app that bundles the cloud storage), how many platforms will I have to develop for (Windows, Linux, Mac, iOS, Android?) and will they promise to never complain when an OS update or API change breaks the app?
We did that for decades. Many products still do. The model is plenty well proven, and the markets well demonstrated, so this kind of rhetorical “well what do you expect?” stuff is unconstructive.
There are reasons a business may prefer to provide their product as an SaaS, but it’s hardly some necessity.
We did cloud backup as a single-purchase pluggable-into-any-provider desktop application for decades? I don't recall anything like that.
The features OP is asking for in their not-SaaS product wishlist just plain didn't exist before SaaS. There was no cloud backup, there was no automatic account connection in your budgeting app. It's not obvious to me that things that require a connection to an always-on server (and some features really do require that) could be sustainably delivered in a one-time purchase model.
Those decades didn't include the features, functionality, scalability, or user-friendliness required by the BILLIONS of people online now using services that weren't able to use them when they required you to do your own setup.
If that's the case you should be able to build a non SaaS competitor and take their market share. In the end if SaaS is really bad for the end user, competition should take care of that should it not?
Yes, but that market mostly existed before everyone expected every app to be online and connected to other services, and since then the market has moved to SaaS. So if customers will pay more for a subscription that is inclusive of these details and maintenance, why would I choose the option that leaves me with less revenue and fewer customers?
What’s the market size for users who will set this up themselves (versus going for a competing app that bundles the cloud storage), how many platforms will I have to develop for (Windows, Linux, Mac, iOS, Android?) and will they promise to never complain when an OS update or API change breaks the app?