An evolving language is one thing. Pre-1.0 was more like many stages of violent metamorphasis. Some changes to the language required changing almost every line of code in the tree. Normal technical debt is when you find an old approach doesn't work any more and you need to fix it. Much of rustc's technical debt is from things like features disappearing from underneath it, or things designed before the features that you would use now even existed.
Looking at rustc as a regular large Rust project is unfair. A new project can at least assume that the features you are using today will exist tomorrow and design with that in mind. Your designs may become obsolete due to new features, but you aren't going to have to pick between a hacky workaround and a redesign/rewrite of 1000's of lines of code.
> And presumably Rust will continue to evolve, will it not?
Right, but it will evolve in a backwards-compatible way, rather than in a "the fundamental semantics of the language have changed Yet Again" kind of way.