I’ve been compiling apps depending on newer Qt and/or kdelibs versions for ages (back when the repository was literally called “kdelibs”, about 20 years ago).

This has never been an issue for me. Even with autoconf/automake, I just compiled everything to its own prefix, so it doesn’t interfere with the system at all. You don’t even need to fix the build system in the cases where it’s broken/lacks features, if you leverage all the “path” variables (CPATH, LIBRARY_PATH, LD_LIBRARY_PATH, PKG_CONFIG_PATH, etc.). But autotools, cmake, qmake, and every build system I’ve used so far supports this out of the box.

Not claiming it’s a skill issue, but I have to say I’m very surprised by reading any of this.

Specifically, for Debian, I was told 20 years ago by a very wise person “you never do make install on Debian, specially not for the kernel”, and taught me how to use make-kpkg (or something like that, I don’t remember the name of the tool), which was a way to make a debian package of a self built kernel, which is obviously something that can’t be installed to its own prefix.