Polaris and Poseidon, Atlas, Titan I & II, Minuteman I, II & III, B58 Hustler, B52 & Hound Dog, FB111 all made it into service in the timeframe that the XB-70 was being developed and planned to be deployed. The US nuclear forces never missed a beat with it's cancellation, nor the cancellation of Skybolt, because of the great depth of resources in the US during the 60s.
This is unlike the situation in the UK during the same period, where the cancellation of a number of key projects effectively ended Britain's career as a world power. The same thing is happening to the US in the last 20 or so years, although not nearly as bad and not crippling per se but the US does have an uncomfortable reliance on equipment that was developed in the 80s and even the 60s.