Firstly, because it would have looked bad and been unpopular with the public to have announced that all on train staff are being sacked. And secondly because it would have been mightily expensive to make the whole lot redundant in one go.
Give it a few years, demoralise and grind people down as much as possible. A good few will leave, some will retire (particularly the ex-BR ones who would be very pricey to pay off), a small minority may move to other useful roles within the company (get jobs as Drivers, etc). Agree favourable starting terms with Aslef, which then pave the way to make less and less effort to maintain the establishment of OBS, eventually running more and more trains without them until the Drivers accept it as normal and become disinterested, finally sealing the deal with a healthy pay rise at some point in the future. In the meantime, employ new-intake OBS on crap terms and conditions which make them easily disposed of. The RMT might object. Who cares. Finally, roll out this model across the network, and award Horton a Knighthood for 'services to the railway'.
Next up, Drivers..
Some people think that the staff and their unions are living in the past. Yet, ironically, those same people also seem to think that this Victorian approach to the business of employing people is 'modern' and 'the future'. How naive some people are.