I don't think it's a case of anyone agreeing to let TAG do something... I think there is an agreement made between the sides on what to do with a surplus. The TU gets some of it's requests granted and so do management.
We get increased benefits, they get to stop making payments into the scheme for a while.
To me it seems no more of a threat than the last time the employers contribution rate was reduced. The only difference is this in a temporary saving.
Don't forget that those negotiating on the TU side have a pension due to them too. Do you think they'd put it at risk?