Just because you give up the SSBNs doesn't mean giving up nukes. Instead of replacing them the money might be better spent on SSNs with nuclear tipped cruise missiles. That would give them more flexibility.
Actually one of the options for Trident replacement is pretty much like what you said!!It involves modifying soon to be built Astute Class SSNs with a small VLS section to carry N-tipped cruise missiles or the new IRBM which the USN appears interested in.