Almost any significant F-22 engine or avionics upgrade would involve so much work, new technology, and R&D that it simply be a new aircraft. It got caught in a technological and production dead end and most of its subcontractors don't exist any more. The F-15EX works because it *is* a new aircraft, even if the original airframe design is old. As someone noted, it has been in continual production with incremental improvements for four decades. I don't think it would be especially cost effective to upgrade the C's to to EX standard either, hence new builds.