> is there a strong difference between dependency and runtime environment?

Programs should rely on the global runtime environment as little as possible

> if sensible people does not bundle the whole python distribution to a "stuff.py"

Unfortunately Python deployment is such a such an unmitigated disaster that it's a leading cause of Docker images.

Deploying a portable copy of Python is about 9 megabytes compressed. This is significantly preferable to multi-gigabyte Docker images.

> people are quick to declare that appX and appZ are incompatibe as they can not run on the same system due to "conflicting dependencies". but who said you have to seach libY in /usr/lib*/libY.so? if you need different versions of a lib, just install them in separate dirs and make your apps find the right one (eg. by setting RPATH or versioned .so filenames).

You make a strong and compelling argument as to why programs should bundle their dependencies and not rely on the system environment.

Users should not have to perform any witchcraft to launch a program. Download and run. No further steps should be necessary.