Nothing. These are rules of the Senate so they can be changed at will whenever they choose. Well, within the rules of the Senate that covers changing the rules of the Senate.

But since it was done, it'll be easier to repeat it in the future. Which was why most wanted to avoid doing it over the years.

And raising the standard of changing the rules just before you leave power.

The fact that the Senate rules are adopted for each new session, at the opening of the session, by a simple majority vote stops that from being meaningful as a tactic (nothing stops them from doing it, but it doesn't do any good.)

They could do that, but there is also no reason that Republicans, should they take power, would just revert back again, and potentially go farther.

Exactly. There was always a "gentlemans' agreement" not to abuse the game, but that decorum has been abandoned.

This is clearly the case. There is dispute over who abandoned what and when.