The end result is being completely unaffected by version changes, the trends in programming that go up and down (javascript, python, whatever, doesn't matter to me what's happening in their world).
It helps that I was also an early adopter of Rust from pre-1.0 in college around 2011, and so I've had a relatively stable implementation language. That combined with C and Luajit is my permanent stack right now, but I'm in the progress of migrating to my own programming language.
The natural terminus of the path you are suggesting is to reimplement everything. Be prepared for a long road, even with LLMs at your disposal now. An LLM can't design what you want because it can't read your mind.
What’s the best way to get started with Common Lisp now for a very very experienced programmer?
But I'm the sort of person who shrugs when someone says "this software is not updated anymore, it must be out of date." I say: maybe that's good, cat(1) hasn't been updated much, isn't that good?
And CL is just an absolute joy to develop with. Definitely my "stranded on an island" language.
GUI are useful, but I spent so much time trying to find the perfect ones while I could quickly draft a quick CLI script for my specific problems. These days, I focus on finding libraries and utilities that solves the core of my problem, then wire them up in a way that solves my workflow. After a while, you just find that most of your computing needs are solved.