A permanent option just isn't scalable.
I compared Construct to GSM2, a like-for-like featured GSM2 costs about the same as 6.5 years of Construct; that's a pretty good deal in my opinion, especially considering that most game projects can be wrapped up in less time, meaning you wouldn't need to spend as much if you went with Construct.
Would I like to see improvements? Sure - I'd love to see a sliding scale discount (i.e, buy 3 years up front for a 30% discount), it'd be very interesting from a business standpoint to know Constructs retainment rate and if such a change would boost revenue; but I can see how that would quickly create issues for Scirra.