U.S. ballistic-missile subs might become targetable while at sea.

I stopped reading right there.

I assume this is directed at China since they are the only true threat to USN. They have 12 nuclear subs (6 SSN attack subs + 6 SSBN). The 6 SSN can venture out into the pacific and Indian ocean to hunt for US SSBNs but they won't cause they will be busy protecting their 6 SSBNs. That leaves the 30+ Chinese SSKs diesel subs which can venture out into the pacific but they will be constraint by range and other limitations so they won't be a threat either. They will be busy preventing US from CSG from approaching Chinese waters and thwarting any naval blockades.

So what is the logic in throwing more money at silos? If money needs to be thrown at US nuclear triad, then it should be towards SSBNs and bombers (more so of the latter). It's a big ocean. SSBN can surface at any location. Release their punishing payloads and descend back into depths. Nuclear silos will be maintenance nightmare and worse are fixed locations. If they were mobile, that might make somewhat attractive.