We do take compatibility seriously. Others have given examples in this thread how we do this technically. I don't think any design process can guarantee perfection at first try, hence we prefer steady improvement over stagnation. When we decide it's important to change something, there's advance warning through deprecation.
If you took it seriously, you wouldn't break the language with every major release, and you'd spend more time not shipping things that are obviously poorly thought out.
No design process can guarantee perfection at first try, but that means you have to invest the effort to maintain what you produce that's imperfect. Simply accepting that you'll produce garbage is how you produce more garbage.
"Stagnation" is what happens to the hundreds of thousands of lines of code in the world that you bitrot with every breaking changes. When you invest more effort in craftsmanship, you're not stagnating the language.
