Cows Getting Bigger (#20) has the answer, IMHO.
The only people who know where the Trident sub is "on station" at any one time are in the sub - and they're not talking !
You can't retaliate against something when you don't know where it is. Any other delivery system can be pin-pointed, on the ground or in the air.
Admittedly, Trident has really only one function: MAD. But that suffices to protect us from nuclear blackmail. You can have as many other nuclear ideas as you can afford after that.
Perhaps I really
am going gaga ?
D.