LLMs have meaningfully decreased the cost of rewrites, such that many rewrites that wouldn't previously have made sense now do. They are still significantly costly and often a bad idea.

An absolute doddle assuming you have an absolutely excellent test suite. Otherwise on any nontrivial codebase you will still have regressions that make it to production and you will waste your time squashing bugs rather than writing new features.

it is sort of amazing what I think is practical.

"rewrite in python." "no, let's try rust."

"I don't like the grand piano in the basement, let's see how it looks upstairs"

As an American English speaker, I'm going to assume "doddle" is a synonym for quagmire.