Once upon a time someone came up with copyright protection, and now every big company abuses it to ludicrous points, especially the ones from Asian regions, who would sue you if the word Goku or Vegeta even dared appear once on some random video for a few seconds, if they could, but because they can't they'll just take the video down. So the way copyright works right now, that thing with valve buying rights could never happen. Hell they might risk getting in some minor troubles themselves for it.
What ESF devs are doing now would be in corporation theory a terrible crime, because I assume nobody here has ownership of any of the playable characters. No the only thing they would get is a terrible lawsuit if god forbid something really unexpected happens.
Note: You don't want valve to buy ESF anyway, because then you would have to pay for it. Here it's free. Also if somehow it get's in the hands of Namecake Bandages(cause they do most of these type of games) it will be abolished from the PC and either erased from history or just be turned into a mere console game.