Many opensource projects are in same state, I know for sure my projects become spaghetti if I work more than a year on them.
Besides, I’d argue that if you need to rewrite (part of) it is because it wasn’t maintainable in the first place.
Well, if rewriting is maintaining, everybody can write maintenable code.
Did it become a mess? Rewrite time!
For me the art is writing it so you don’t need to rewrite and you don’t need a janky temporary permanent workaround if requirements change. Clean interfaces, SOLID, plug-ins, etc. Can’t do it myself, but the legendary 10x devs usually do.
All code is maintainable with enough time and money.
But yes, well structured code where those rewrites are minimal is the goal.
There probably is a threshold where the amount you have to rewrite becomes too high. But with each rewrite, hopefully the next time you have a section you need to redo its smaller that before. Eventually going from rewriting a couple thousands of lines to just a hundred or so on the 5th iteration.
The most maintainable code is built to be replaced with minimal impact.
How much of the program will must be replaced if you remove one module? If you need to replace the entire program, then your program is not maintainable. Too much is heavily dependent on this module.