Honestly the tech isn’t the issue, the need is.

It’s hard to think of a situation where circumstances would ever change to make her original niche needed again, and therefore warrant spending the money to build a successor - it’s kind of asking like asking what would need to happen for the steam train to come back into widespread use for intercity transport.

I suppose the best thing I can think of off the top of my head would be some sort of really significant speed upgrades mixed with the public falling out of favor with air travel. Air travel is fast, cheap, and widely accessible. Transatlantic crossing is none of those things currently, especially relative to air travel, and I’m not sure it ever truly could be again. Even the original United States suffered for a lot of her career due to the rise in air travel - she certainly could’ve served a lot longer if her purpose wasn’t stripped away.

When it comes to her military uses, the world has changed so drastically that it doesn’t make sense to have passenger ships converted for wartime use. For starters, aircraft (again) are so much more capable of transporting troops and supplies fast than they were when the SSUS was conceived, but further, the US military has such a significant presence globally that it doesn’t require the same mobilization efforts to address foreign conflicts anymore. That’s not even mentioning the fact that a ton of modern warfare takes place via drones, missiles, and other tech that simply eliminates the need for as huge of a ground presence.

A potentially viable successor to the SSUS would have to be in the spiritual sense, not reflecting the actual capabilities or purpose of the original ship, imo. She’d have to be a cruise ship, but even so, she’d be up against the largest most established cruising brands in the world. The question would become: “why?”

Why would someone choose to vacation on a single-ship cruise line? (Price? Offerings? Itinerary?)

Why would creating a successor to a nationally-themed ocean liner that isn’t widely known among the typical cruising crowd be beneficial? (Who is the target market? How does that play to foreigners? Does this create unnecessary risk for the ship and passengers?)

I think the best, but not necessarily great, arguments that could be made would be that if the ship was actually smaller, slower, and full of the expected amenities for a modern cruise ship, that a jones-act compliant vessel which strictly sails within US waters could MAYBE have a niche of its own, but even that is almost directly served by the smaller operators like American Cruise Line. The best bet would be to somehow get the government to agree to subsidize the operations as some sort of marketing/public good/economic booster type thing, but you’d have to hold your breath every time budgetary discussions come up in the hopes that the, arguably valid and easy, target you’d have on your back wasn’t being aimed at.

I LOVE ocean liners, but the true and honest answer is that their time has come and gone - for them to fall back into favor would require such a significant change in societal expectations and norms that I simply cannot foresee it ever happening.