In reply to tjpa.

Yes, that is right. The current amount of old themes is currently too big. I've spent HOURS browsing for themes only to find abandoned ones, broken ones, incompatible ones.

I bet that if WP team really wanted to delete all the themes updated before 1/1/2013, this 24% could easily go up to 56%.

I'd like to see the results of a rerun of this study but filtering out all those old themes, which barely work with WP 4.2.

Also, a monthly rerun to build a report on how is this improving would be very interesting too.