I'm not sure "replaced" so much as "didn't grow" or "didn't grow as fast". Even in the 90s when the US DOD mandated Ada it wasn't used universally in defense projects because people got waivers for it. As the number of projects grow if Ada's not growing as fast as the others in adoption then it's getting a smaller and smaller share of the market. C and C++ took a lot of the marketshare in newer developments and rewrites of older Fortran, JOVIAL, and assembly systems.

So as a percentage of the market in the aerospace, defense, and safety space it has shrunk. In absolute numbers, I'm not sure if there are more or fewer systems developed and currently being maintained in it than 20 or 25 years ago. But C and C++ have definitely grown in that space.

Thanks, the usual sentiment with C and C++ is that it's unsafe to make anything beyond a hello world. Happy to hear that defense is using them. Last time I had to write Ada was in university!