I honestly believe the legacy fleet as it is now refered (sic) to will be around for quite a while yet.
That has always been the case. The company's projections at the last Investor Day was that Mixed Fleet would constitute 40% of crew in ten years' time.
It is only BASSA's particular brand of fear and paranoia that is responsible for the suggestion that existing crews will be forced on to Mixed Fleet or starved of work.
What Mixed Fleet does bring is internal competition at LHR and a bit of healthy competition is no bad thing at all.