A few subscription proponents mention the industry is moving towards this model, but besides Adobe and Buildbox, I'm not aware of too many lockout-type subscriptions. Would this even be on the table if it weren't for them, or is it inevitable?
Shoutout to newt and Bleenx
I don't know about Buildbox, but sure, Adobe had a big hand in it all. But so did many others such as Autodesk. If you subscribe to use Max or Maya per month/year, and then you cancel your sub, then you can't edit your models or animations until you subscribe again. It's the same with Adobe Photoshop files and such. I still use Adobe, but as a 3D animator, I no longer use Maya unless I'm told I have to by a client. Otherwise, I use Blender.
The subscription model is inevitable. It's a guaranteed way for developers to maintain constant revenue and to be paid for updates rather than release them for free every year, much like Scirra has done with C2. Those same companies also use the subscription model as a way to stop piracy of their programs, though that's another conversation since pirates wouldn't buy it anyway and still find ways to crack subscription-based software.
Unity has a nice subscription model: pay as you go. But in the end, sure you own it, but you won't get any of the new features and updates after that to my knowledge. It's a catch 22. Thankfully, they still offer the free personal edition, which I believe many game engine companies also provide, and they're all limited.
tl;dr Yes, it's inevitable. I'm sure YoYo Games, Clickteam and others will follow suit if they see sales dips and work hard to provide updates with little payoff.