: Putting it on Steam would get a ton of people to buy it, not even the people
: who are just playing it now, but imagine it being in a steam sale. A ton
: of people would buy it just because "oh look halo".

Haven't a few games gone 'open source' over Steam as well? Could do that. Buying the game on Steam gives you the privilege to mess with the code to the engine. Code could be distributed over Steam, or probably using a similar model/method that UE4 uses for providing code access to licensees. They'd probably, understandably, require that the code only ever be used for Windows platforms. Dreamers can dream, but beggars can't be choosers :p.