No way an IT system split costs billions unless the person issuing the bill is taking you for a ride.
As I said before something been difficult or even expensive should not be a valid reason for changing a regulatory decision.
I see one of 3 ways forward.
1 - keep the status quo, I think this is unacceptable, but I know many dont agree with me. BT are a very anti investment company. I predicted 2 years ago that g.fast would be cabinet only and was mocked for suggesting such a thing, but it was a given with BT's history.
2 - Keep openreach as part of BT but have it deal with consumers independently of CPs. This I would guess is cheaper and easier than #1, but has not been considered because ofcom allow CPs to tell it what to do.
3 - split off openreach, this we already know is not a proper split but just trying to make openreach untied from BT in terms of decision making and resources. As expected BT are extremely opposed to this which of course has led to scaremongering been spread in the press and a few people on forums.
In regards to the extra costs, there will be some in the short term, but of course no longer will BT retail be able to use openreach income to subsidise its operations and as such openreach will probably have a net cashflow gain. Tied up capex for 5 years? What have BT promised in that time? a very limited g.fast rollout, Hardly much to lose.