Well if the Beeb are prepared to pay that much to hold onto presenters, then one can assume other broadcasters would want to offer close to that?
I agree it's stupid money to offer an news anchor, especially when there's still a definite pay gap between men and women in broadcasting. But if ITN or Sky is prepared to offer £250k a year, then clearly the BBC feels they have to make an offer too good to turn down.
News broadcasting relies heavily on stalwarts; people connect better to familiar faces. There's a case to be made to bring in fresh faces, and even allow them to move on if they get a suitable offer. But I expect the BBC don't want to be seen as either a stepping stone, nor a revolving door. And in the great scheme of things, there are only so many broadcasters, only so many news/current affairs programmes. The market for presenters, at least on national networks, may well be saturated at this point. Probably very hard to break into.
I'm neither yay or nay on the issue. Just speculating.