There's no need for rouge to do that, they could just have a sub-fleet of 330s that were only PY/Y. I'm not sure how much they really need that though. Some former rouge destinations seem to be doing very well as mainline, e.g. ATH, and the XLRs will give them the ability to target many lower-volume routes while still offering a proper J product. And they can use the 7M8 for a few things like KEF and DUB.

As for overhead, rouge exists, but there are surely ongoing costs to maintaining it. I don't know what the annual paperwork and so on for an AOC is, but there will be some. Plus some duplication of staff, redundant procedures, etc, that could be eliminated. And greater flexibility in scheduling and moving assets and personnel around day-to-day.