I think you're directionally correct that military spending could be much lower and more effective but the notion of relying on a single sub as a deterrent is f

I think you're directionally correct that military spending could be much lower and more effective but the notion of relying on a single sub as a deterrent is far too extreme. In that scenario an enemy only needs to compromise a single sub and your entire deterrent is gone. That's not a stable equilibrium. The whole principle of a triad is based on the expectation that an adversary could compromise your retaliatory ability to a high degree so you need to mantain robust options.