A wonderful property of lisps is that you can usually parse unfinished code into raw sexps. This is really handy when building editor tools.
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ASTs typically discard comments, and that's usually what you want.
The only time (AFAICS) that preserving comments is useful is for writing a code formatter.
Could you write a formatter in terms of a list of lexemes? A CST is a non-trivial bit of code for one use case.
Over a sufficiently long time horizon, all code you write is legacy code.
LLMs seem to handle dependency upgrades really well.
The task is well-specified, there's usually a build/test suite to check correctness of the modifications, and there's often a changelog they can consume too.