I think the tried and true “Your salary offer is too low compared to your competitors, or what I can do for myself” is pretty much fine right?
Corporations have enough staff that they can lay off hundreds or thousands of workers without a single worry. They can hold out not paying you the rate you want for much longer than you can afford not paying your bills and eating. Eventually, you will accept a lower salary and if you don't there are plenty of others who will. The vast majority of all employed people have next to no leverage compared to the companies that hire them and companies know it.
The large problem with the collusion before was that the 3-4 that already paid top of the market salaries colluded to keep them the same. It doesn’t mean they weren’t still top of the market for someone with a given level of experience.