At my old company we had an innovation consultant come in and tell us that kind of approach was nearly impossible to manage in the longer term. Apparently teams dedicated to holding the fort grew bitter and had much lower retention than teams on greenfield work.

Paying back the debt, he said, usually requires too much knowledge of a system to cycle teams in and out consistently enough to ensure everyone gets a piece of the new versus the old to keep them engaged.

Whether or not that was backed up by hard data, I'm not sure. It was an interesting angle I hadn't previously thought of though.