Once players have left they probably not coming back to the current system. So it could take a decade or so to rebuild afterwards.
Depends how long the financiers are happy to throw 10 of millions at a product no-one watches.
If, after 3 years, the fan base is about the same as the Championship in England, or MLR in USA - but costing a hundred times as much; and the breakaway has ceased to exist; then I would imagine that those players who broke away would return to actual professional rugby, rather that find new jobs outside of the sport because they're sulking.
For rugby fans who are already fans - we have existing allegiances, and are unlikely to watch this as we'd be neutrals for every single match, and there will still be plenty of matches to watch, for which we're not neutrals. I'd expect the viewing figures to be broadly similar to viewing figures for the T14 as shown in Australia, the URC as shown in NZ, the Premiership as shown in SA or Super rugby as shown in Europe - ie. negligible.
But it'll attract new fans, because it'll have big names they've never heard of, because they're not existing fans. Non-rugby fans who occasionally watch rugby are vast in number, but the clue is that they're non-rugby fans who occasionally watch rugby. That's not going to suddenly change because there's a new league catering for people who want to watch rugby every weekend for half the year, with names they already choose not to watch play matches for teams that don't mean anything to them.
Sport is built on tribalism, history, allegiances and rivalries. Money doesn't build those, time does, eyeballs watching does.
Exeter were promoted into the premiership in 2010 - they still don't really have a rival. They're no-one's "we gotta beat these ba
stards" team (though they did have tall-poppy syndrome for a bit, and a bit of racism to dislike, and some weird covid conspiracy theorists to take offence at for a bit).
No-one really cares about the Welsh regions - and they're actually regional and Welsh, not entirely plastic teams with no location for a support base.