True, tho that's only one of many ways to get locked-in. Just to name three:

1. what if I want to switch platforms?

2. what if they switch business models?

3. what if they go away?

The only real concern is #1 with Sketch, and that's your (org's) choice so you do have some choice in the matter. Since the format is well-documented there's ways to move away if #2 and you still have access to existing software with #3 (save issues with a licensing server perhaps).

Figma is a concern with all three unfortunately. Everything I see about its capabilities is amazing, but I've resisted for this reason. Hopefully they open their format.

Just because it’s Mac only doesn’t mean you don’t own your own files. If you’re going to take it that far, being computer-only is a form of lock in too.

Having said that, there are windows apps that can work with sketch files. For example https://icons8.com/lunacy