Then you have to let big corporations like Google (*) encourage you.

As to Sailfish/Jolla's business model, it encompasses more than this.

(*) actually only that one - even Apple's iOS works "on only one somewhat obscure brand of phones".

I dunno. Quite a lot of the apps are proprietary.

Undeniably.
But their business model revolves around making people pay for the OS, not around making it a carrier for a data mining ecosystem.
The licensing makes it fairly transparent, and I must admit (of course that's just a personal thing) I trust Jolla more than, say, Google/Alphabet, a company that completely revolves around the data collected from me.
Have another look at their licenses and make your own decisions:
And, I cannot stress this enough, it's build up like a real Linux distro, and it works that way too - I can apply already aquired knowledge, I (can) have full control over the system.