All coins have 2 sides. Paying once is great, and if uou are happy then that's good. I use a bunch of software purchased on this model.
It's worth understanding though that the seller owes you nothing after this. Not support. Not development. Not keeping up with API or whatever changes. Not bug fixes (although most everyone ships some free updates for a while.) Obviously not hosting.
And for lots of software this model is perfect. For other stuff it's imperfect (as anyone who desperately needs Google Support will sympathize with.)
You get to choose what software I'd on your business-critical path, and what is on your "can replace anytime" path.
Already I can see several SaaS subscriptions that can be replaced here.