Then 1SL would be held to account by the SoS for the decisions he makes, about money he owns, for capability he is to provide.
IF 1SL decides that he can do all the stuff an MPA can/should do, but with something else, he makes the case, elucidates the risk and will carry the can. If the SSBN is compromised, or an MCT event fails, and that risk is realised, then he gets the blame.
At the moment, all that happens is no-one is really blamed, and no one is held to account.
It's exactly the same for things like helicopters - CAS will be held to account to provide a certain capability. If he thinks he can do that with a load of Puma (or whatever), and invests the rest in shiny fast jets, that's great. But the day he can't provide the capability then he gets a kick up the pants.
H2A (as the MoD buzz-word bingo has it) is a great thing, as long as it's done properly. Unfortunately there are some toes that are going cold in 'Town' about it all, so expect to see fudges galore.......