I see people sharing here how they don't get raises, even when promised. Yet they are offered an increase to stay. That offer is often less of an increase than what they will get at their new job.
We are gaining the upper hand here. They have been throwing their weight around for too long. Let's throw ours around.
Instead of rejecting their offer, counter-offer with demanding a BETTER THAN YOUR NEW JOB'S increase retroactive to a specific date. That date can be any one of your choosing. It makes more sense to associate it with them hiring somebody new for more money or the first time you were denied an increase.
They may balk at this negotiation because they're still getting used to the shift in power. That's not your problem. They created this situation and their effort to resolve it says a lot about them. You should still not accept any offer that comes after resigning. Share these offers with the peers that you're leaving behind. Also twist the knife. Be sure to refer their best people to your new job and any other good offers that you declined.
In 2017 my employer of four years tried lowering my compensation target despite four years of positive employee reviews. I moved teams to get an increase and eventually left in 2020. A month ago the same manager from 2017 offered me double that 2017 target to return.