Changing from $99 every two years to $99 every year is not a change for the users. Also, Sketch has never had much regard for backwards compatibility, so this is a de-facto subscription product if you work with more people.
Basically going the Adobe way — for a large group of users that actually switched to Sketch for this very reason...
The stick-to-your-old-version if you want to avoid that fee is, imho, a terrible argument for not calling it a subscription.