Is such a strategy really feasible? Adding legislation that a game has to be made operable in a reasonable manner after the publisher discontinues support for it in no way influences this strategy.

If someone wanted to do such elaborate botnet defamation attacks in hopes of getting the game playable on 3rd party servers they could've done that already without legislation.

Bots making the game unplayable is a problem, but opening the servers in general would help the problem as private servers can implement harsher requirements for players than official ones usually do, opting to rather make a huge bot-filled cesspool as you've already said.

However, this proposal isn't a general "all games must have FOSS self-hostable servers" proposal. It's just a "if you kill a game it still has to be alive afterwards" proposal. Whether publishers open servers or not before they shut theirs down is their decision without the proposal as much as it is with it.