If you can’t control the abuse and you don’t want to do anything about it: do the thing nobody will like but will solve the problem: kill the damn game already if you don’t want to support it. That’s what I expect to happen before they face this issue, they’ll shut down official servers and make the game unavailable before they try to solve the bot problem for a game so old.
Nobody would benefit from that. There’s literally a huge movement on right now to stop companies from doing exactly that with the Stop Killing Games initiative. It’s far better for Valve to just announce the game is EOL and no longer receiving official support but to leave it in exactly its current state.