> There’s nothing stopping anyone from going back and using all of that old software exclusively.

You need to search for it on old ftp sites.

> For some reason everyone prefers the newer software, though. Perhaps there’s more to it than binary size?

The compilers have "evolved". Compiling old code is challenging, to say the least. I do compile old programs when the new seem to explode: xpdf, xsnow.

Compiling old compilers is impossible because they rely on ancient kernel headers.