I do have control: If femlisp or any other library makes a boneheaded decision that breaks my software, I can stop using it.
yes, resolving that is trivial -- once you have identified the problem.
regarding the recent discussions i'm generally baffled why it is at all a question whether to make a build software deterministic or not. in my view if there's anything in the global state that has an effect on the building of a software, anything, then it's a bug.
and if this state can be set in a way that it leaks out and influences builds later in time, then it's an ugly bug feasting on programmer nerves and time.
the only question to me is what should be the defaults, and how can the transition be made smoother with deprecation warnings and whatnot.
and if there are some unmaintained, bitrotten libraries and some software depending on them, then they have all the freedom in the world not to upgrade ASDF, or to put on the maintainer hat and patch the bitrotten libs.
by now the time on spent discussing this would have easily been enough to fix all of them twice over, and to add a section to the top of the manual, with bold, that lists the global state that ASDF guarantees and isolates.