In the specific case of the A-10, it was designed for a world that had extremely primitive sensor systems that couldn’t even reliably lock onto a plane when looking straight at it. We aren’t in that world anymore. Not only is the terminal guidance sophisticated and reliable but the warheads are designed specifically to destroy aircraft like the A-10. Everyone has had 50 years to work on that problem.
Modern Russian and Chinese systems effortlessly deal with an A-10; it lacks the power systems to support modern counter-measures against modern systems. It isn’t upgradable like that because that wasn’t a design objective.
Modern military aircraft like the F-35 have critical combat roles that don’t involve dogfighting. Their sensor suites and processing capabilities are at least as important as any other particular feature, and other aircraft don’t have that. Their systems can measure and exploit their environment to an astonishing degree.