To be fair, it is described as "Expires in 6 months" on the purchase page which makes it very unclear if the binary you downloaded is going to stop working. If it does stop working then it's not unlike subscription software.

I was actually vaguely contemplating purchasing it until that uncertainty made me think paying them might give me something worse than free (ie: something that would break at an inopportune moment in 6 months).

I can confirm that the binaries do not stop working after the 6 month.