How about if valve has an agreement with developers where if they release a game in early access, and then abandon it, it switches to free-to-play?

Developers don't have to pay for the servers, and the source code isn't released. Gamers get to still play it, if it had any redeeming qualities, or for nostalgia. And valve gets...uhh... community goodwill? Honestly the lack of benefits for valve is probably why this will never happen.