MV3 is basically MV2 with one tectonic change: replacing persistent background pages with ephemeral service workers. So a large part of this effort actually DID go into MV2.

There's a better question: what if instead of investing huge amount of times into DNR they put it somewhere else? For instance, into providing better tools for extensions to persist their state so that we didn't have to rewrite the extensions from scratch to make them work with the new service workers model? I think it would've been much much much better. Unfortunately, that's how hindsight works.