Rewrites are only cheap if the rewrite solves the problem.
And the rewrite can only reliably solve the problem if you understand the problem, and even then it's obviously not a guarantee.
If you have a huge blob of code that nobody understands then re-generating a new blob of code that nobody understands is unlikely to solve the problem.
That said: I've always been a fan of optimistic rather than pessimistic merging, at least for human-generated code.
And the rewrite can only reliably solve the problem if you understand the problem, and even then it's obviously not a guarantee.
If you have a huge blob of code that nobody understands then re-generating a new blob of code that nobody understands is unlikely to solve the problem.
That said: I've always been a fan of optimistic rather than pessimistic merging, at least for human-generated code.