They aren't the sole trump card, that role is also played by manned bombers that can launch on warning from the middle of the without committing to bomb that targets, without even having the ability to arm their warheads (https://en.wikipedia.org/wiki/Permissive_Action_Link ); SLBMs are trump cards against undetectable attacks, or at least ones that can be detected in time, see for example https://en.wikipedia.org/wiki/Fractional_Orbital_Bombardment...

You'd also have to be very confident in your ability to carry out a first strike. Ah, and there's a quick fix, a limited ABM deployment (we've got them and are pretty confident they work). Since you can't pick which warheads get taken out by the ABM systems, the difficultly becomes massively greater.

Note also that historically only the US believed in MAD, the Soviets correctly thought it was unmitigated evil. Then again, they had to accept it....