The only reason I no longer pay for a license to Sublime Text is that it's not available on my OS. Them either supporting more OSes (a lot of work!) or making it open source would at least cause _me_ to buy a license again.
They don't even have to make it "open source", keeping a proprietary license such that it's just "source available" would be fine with me too.
Edit: Hilarious to get down voted for this comment.