Grady Booch said that any large system that works is invariably found to have evolved from a smaller system that worked. I've seen this cited as Gall's Law, from John Gall's 2012 book Systemantics, but I read it in a book by Booch back in the late 80's/early 90's. At that time the "waterfall model" was the conventional wisdom: to the extent possible, gather all the requirements, then do all the design, then do all the coding, then do all the testing, doing the minimum of rework at each step.

It didn't work, even for the "large" systems of that time: and Booch had worked on more than a few. The kind of "system" the OP is describing is vastly larger, and vastly more complex. Even if you could successfully apply the waterfall model to a system built over two or three years, you certainly can't for a system of systems built over 50 years: the needs of the enterprise are evolving, the software environment is evolving, the hardware platform is evolving.

What you can do, if you're willing to pay for it, is ruthlessly attack technical debt across your system of systems as a disciplined, on-going activity. Good luck with that.