If you design a long lived code cleanup project where the only option is "we can't stop this project for another 6 months or we'll have an even bigger mess on our hands," that's poor project design. In this way, rewrites are almost safer. They're usually "atomic" and you can "roll back" by throwing it out if you can't focus on them.

All this is based on dynamic business and product needs that are shifting every couple of quarters, and a technical team that's getting new people, and shifting off more experienced folks every couple years. That seems to be a pretty standard expectation to me, though I'll admit I'm mostly looking at this through the lens of early, mid, and late stage startups. It may be big companies that move slowly and can guarantee resources for a larger project for a long time are a different story.