I really don't think it would destroy Welsh rugby. Rugby runs very deep in Wales and a year or two out of the six nations isn't about to ruin that, it might actually give more limelight to ERC teams being able to play in Wales. It might also give a shake up to the WRU that is sorely needed.
I'd still be a fan of promotion/relegation even if Wales were knocked down as it is not fair for teams to retain their place in the 6 nations if there are better teams in the ERC. It was never about bashing Italy.
Saying that I believe there should be a promotion/relegation match each year with the defending 6 nations team at home. So that the best ERC team would have to prove they are better without any doubt.