"All agencies will fully implement updated policies, including ending the optional 12-month embargo, no later than December 31, 2025." Why does this need a 3 year transition period? Six months would be plenty.
Academics must still publish in the same prestige journals as before to earn merits for jobs, promotions, grants, and prizes. Those journals are largely published by for-profit publishers that want their money one way or another. If their subscription revenues will be lower, they will want more money from open access fees. While subscriptions were usually paid by university libraries that receive their funding from various sources, open access fees are often the responsibility of the individual PI.
Some universities have agreements with some publishers that the library will pay open access fees for their researchers. Others will try to negotiate them, but negotiations take time. When there are no such agreements, the PI must pay the open access fees from their grants. That means grant agencies must establish policies on how much funding to include for that in their grants, and the money has to come from somewhere. The agencies must decide whether to reduce the number of grants or the amount of money available for other purposes. They may also request more funding from the Congress, but that takes a lot of time and the outcome is uncertain.