Blocktrades is a guy called Dan Notestein - as I understand it he runs a dev company and he has team working on the chain software - the second place GTG I know is very knowledgeable - obviously howo is paid to work on Deving - I'm not sure about the rest - I think a few might get their Trades votes because of what they've developed on Hive - Marky springs to mind - rather than the deeper development.
It's quite possible that some of BlockTrades Dev team are in those T20 witnesses, I don't know how many people he employs, but it would make sense.
But some just seem to be along for the ride, you're right. It's a good idea for a post 'what do the Top 20 witnesses actually do'...?
I mean IF they're not FULL TIME deving Hive, why are they in the T20?
Then again aren't governance and Deving different, the later funded by the DAO?
Oh they all vote for each other, and no I wouldn't expect much investment because of this!