www.makemkv.com

Is there a chance that I buy this software and lose it one day? On the buying page (https://makemkv.com/buy/) it says "However we reserve the right to disable any specific registration key in future versions on extraordinary circumstances." at the bottom of the page...
Am I understanding this correctly that I might lose my account if the creators of MakeMKV decide to do so or if they are forced to do?

Theoretically, yes.

Practically, I doubt it, unless your registration key finds its way into hundreds of machines.

I do not know how many registration keys have been "blocked" at this point. I suspect it is just a few, revolving around a company in China that has "sold" MakeMKV under their own name.