But then one senior developer would claim that they are better or different than every other senior developer, and negotiation would return. Sure, Tim might be willing to accept the average rate of $95, but Jim worked as a staff engineer at Google, and so on.
In general, you would expect salary negotiation to be worse for any given employee, because they have limited tools to research with compared to the hiring company, and far more limited (man) hours to do that research.
I would also imagine negotiation still exists, but now more emphasized on the extra benefits, eg PDOs, medical plans, bonuses, etc.
And ofc the added negative that the company can hide unfair practices, eg underpaying particular groups, or hiring someone's kid for 2x the pay, or whatever.
The main negative is that someone who was skilled at salary negotiations now makes less, but presumably most people aren't skilled, making it more a net positive. The biggest concern tbh is if the whole industry was overpaying its workers due to the fog of war, and this suddenly cleared things up, and employees across the board take a hit
The flipside, which is largely why I am not in favor of legally enforced salary transparency, is that you ask for the salary range, they say $90-100k (or whatever), and you can justify why you should get 100 or 105 or 110 with examples. Or you can ask for $90k because you're happy with that and know you won't price yourself out of the position, or any number of other possibilities.
It just makes the entire process much less flexible, which I know a lot of us developers love, but in the business world it doesn't make much sense.