It looks like they started by selling a Java tool called IntelliJ Renamer. Likely just bootstrapped that and then reinvested the profits.
They only switched over to the SaaS model maybe five or six years ago, before that you'd buy one copy the application and then buy a newer version when you felt like it. Now you're basically buying a new copy every year no matter what, just like everything else.
I've been paying for it for a while and I didn't actually know that. I thought after I stopped paying it would be unlicensed and bricked. Either way though I think the upgrades are probably worth paying for.
I wouldn't say it is SaaS model. You own a licensed copy pinned at version forever without extra pay and also for offline use. I would rather call it annual upgrade subscription software.
They only switched over to the SaaS model maybe five or six years ago, before that you'd buy one copy the application and then buy a newer version when you felt like it. Now you're basically buying a new copy every year no matter what, just like everything else.