And how much leverage do you think those developers had to negotiate the agreement terms?

Probably very little. Are you implying that they were right to have signed agreements with Unity which effectively gives them controlling rights to the developers’ companies?

IANAL. In US contract law, a contract that includes a clause allowing one party to unilaterally change the terms at any time may still be enforceable, but there are limitations. Such clauses are often subject to scrutiny and may be challenged if they are deemed unfair, unconscionable, or against public policy. Courts may consider factors like the balance of power between the parties, the clarity of the clause, and whether there was mutual assent to the changes.

I'm not familiar with the specifics of Unity's usual contracts, but this is the kind of thing that a court might not take Unity's side on.