I think we're just going to have to accept the fact that Union is aggressively targeting the best junior talent in SL and we can't compete financially (especially if the ERU is prepared to supplement club wages).
What the solution to this problem is I'm not sure. But I suspect things will get worse as the crowds fall off in SL (as a result of the seemingly endless economic gloom) and gate receipts take a hit. Union will feel the pinch also. But the ruggeroid fans are probably more affluent and are thus more likely to attend. And the ERU never has a problem filling out Twickenham for the big games.
If this situation continues over the long term perhaps SL might have to think about tinkering with the rules in order to increase the differences between the two sports, thus making a successful League to Union conversion more difficult. I don't know.