50% of the current F22 aircraft are not immediately available for combat missions. This is standard that not all aircraft arre ready, but for the F22 it’s quite high - reasons explained by others in this thread. Prospects for improving the serviceable rates are not high unfortunately.
The main thing the F15 can do that the F22 cannot is to be built for affordable pricing.
The guys at Skunworks did think ahead, but the rate of advancement in technology means that some things are not foreseeable. There is plenty of room in the plane for more add-ons for example. But the core software and systems are not old and can only integrate with the F35 to a certain extent.
At the end of the day, the F22 was the right plane at the wrong time.