Fully professional rugby union is clearly struggling financially. As has been said previously by others on this thread, by Septi principally, the financial outlay for a Premiership side is absolutely massive.
First team squads are huge and are disproportionate to league squads despite the fact they only have to field 2 extra players. Up to 8 substitutes will be used in the Premiership plus the starting 15 (23 compared to 18 in league). The squads need numbers due to the technicality of some of the positions within their code. Tight head props are usually precisely that and nothing else (yes, I know some players can play loose head as well!)
Most clubs have women's teams attached who are also paid these days and the men's first team squad will have a mirror academy squad with lads on academy contracts. As a result Premiership teams and squads are the size of a veritable army - just check out Sale Sharks website as an example.
To add on to VL's previous comments I would have thought that when both Wasps and Worcester hit the rocks 12 months ago that given players lost their jobs as a consequence that some of them might have looked to league to give it a go and to find paid work. It didn't happen. I'm not aware of anyone who made the move to league from either club. They all seemed to find moves to the remaining Premiership clubs or to French teams.
I was surprised a little by that. Was that the fault of the players or was it reluctance on the part of league clubs to take a punt. Maybe Super League clubs simply cannot match Union Premiership club salaries.
So whilst I agree with VL that I would expect it to happen, it hasn't materialised - so far.