That's... not how that works. I take it you're probably more of a frontend person than a backend person by this comment. In the backend world, you usually can't fully and completely replace old systems, you can only replace parts of systems while maintaining full backwards compatibility. The most critical systems in the world -- healthcare, transportation, military, and banking -- all run on mainframes still, for the most part. This is isn't a coincidence. When these systems get migrated, any issues, including issues of backwards compatibility cause people to /DIE/. This isn't an issue of a button being two pixels to the left after you bump frontend platform revs, these systems are relied on for the lives and livelihood of millions of people, every single day.

I am totally with you wishing these systems were more modern, having worked with them extensively, but I'm also realistic about the prospect. If every major airline regulator in the world worked on upgrading their ATC systems to something modern by 2023 standards, and everything went perfectly, we could expect to no longer need backwards compatibility with the old system sometime in 2050, and that's /very/ optimistic. These systems are basically why IBM is still in business, frankly.