Why does the Navy want to replicate the mission of the SSBN’s. I ‘get’ that the “Zummwalts” are probably going to serve-out their lives as test-beds. Maybe they should paint them orange?
Couple of thoughts on this (only half-baked so you get what you pay for...).
The mission is to put ordnance on target, in a hurry. There may not always be Air Force assets within range, or Navy assets (carriers, ships/subs with cruise missiles, etc.) Hence the notion of conventionally armed MRBM/IRBM/ICBMs.
First thought was to re-arm SSBNs with missiles equipped with conventional warheads. That could/should work but... Why move away from it or de-emphasize it? Perhaps the Navy is worried about "near peer" players. Used to be our sonar was so good and adversary's subs were so noisy that a US SSBN would know if anyone was close enough to matter. As potential enemy subs have gotten quieter, they can now be closer to our subs without our SSBNs knowing. Launching a SLBM is a noisy affair and will attract attention. Having enemies potentially closer makes this a much more risky proposition. That may be a risk you're willing to take in something as all-in as a nuclear conflict. However, we may not want to accept that risk to a strategic asset for a regional conflict or target of opportunity.
Developing a surface ship based system that can fire MRBM/IRBM/ICBM ranged weapons increases our strategic options. Once we have the delivery system worked out, who says the warheads have to remain conventional? We do this right, they could go nuclear. This is in line with the Reagan strategy from the 1980s - make your opponent's job so difficult they can't expend the resources to counter it.
Who says the warheads have to come back down? Once you develop a vessel/launch system/vehicle (missile) that's a potent platform for development. Sure, we could put a conventional warhead on there. We could put a nuclear warhead on there. Or we could even put an exo-atmospheric kill vehicle on there and have an even more potent missile defense and/or anti-satellite system. Once you have the fundamental capabilities, it can act as a springboard for a whole family of things.
Speaking of development, this program makes an excellent excuse to (continue?) development of some very accurate re-entry vehicles. When you're sending a few hunderd kT of nuclear bang with air-burst fusing, pinpoint accuracy isn't a requirement. But if you're only sending a couple hundred lbs of HE and a hardened penetrator you've got to get it right. This is basically saying the RV would be a hypersonic maneuvering weapon since it would need to adjust aimpoint on the way in. This may be the Navy's way of saying they want in on this tech development.
Speaking of development - even bigger picture - the fight is moving upstairs. We (humans) had some notion of not militarizing space. Lofty goal (pun intended) but the reality is the fight is moving upstairs. We have a Space Force. We have all kinds of assets in space, so do our adversaries. We have weapons that can reach into space, so do our adversaries. In general, programs and projects that increase our presence-in, knowledge-of, and ability to influence space are a good thing.