I keep hearing this nonsense over and over. I've led two full and very successful software 50-100K LOC rewrites, both with the team and stakeholders who had a lot of experience in the problem space. We replaced unmaintainable systems by a much fresher, MUCH faster codebase. They used largely boring and reliable technology. Rewriting 1%, 10% or even 50% would make no sense and essentially get us little or nothing.

Yes, you need to know what you're doing if you plan to rewrite anything, and make sure that the people who know the little legacy details of your system are around, but it's absolutely the right thing to do in many situations.