Yeah, modern purpose-built cruise ships are just hideous.
They make me think of Rio de Janerio high-rise slums - just built up higher and higher, until a law of physics is breached. This, done on a RORO car-carrier hull. UGGHHH-lee.
The United States was a beautiful ship - designer Gibbs had an eye; he was the right man, at what turned out to be the wrong time to be the ideal ship designer. The US had beautiful lines...and was only in service for 16 years; its last 30 years were in the Delaware River. Fresh water. Not nearly as hard on the hull.
I don't know why it couldn't have been rebuilt as a smaller cruise liner; but probably the reports are correct. Often times it's the same reason why an historic building cannot be rebuilt after a seemingly minor structural failure or a fire...the cost of mitigating all the damage, and then complying with new laws or standards or expectations, is greater than demolishing it. Or just selling the hulk in question for scrap, in this case.
I'd seen this in another waterfront example, much-smaller scale. An area I lived in for years, a tourist area dominated by an inland lake...around the turn of the century (1900) there was a fleet of (relatively) small lake steamers. Not exactly romantic, they were propeller-driven, low-freeboard...utilitarian. They connected the major city at one end of the lake (20k population) with the county seat at the other end; and operated profitably until 1927. Then, unprofitably, as the brothers who owned the line ran them with a sense of civic duty.
And then with the brothers dead, the last surviving boat was run through WWII owned by the city - because gasoline rationing. The steamer in question, ran on coal.
It was run up until 1950, where the city announced plans to stop it. The boat was sold to its last pilot, who also owned a junkyard - for $100. Seriously. It was seen as worth that little.
He ran it a couple years until a navigation incident, on a charter trip after sunset, had the boat sink in shallow water. Pumping it out was no problem, but there was a lawsuit, and that was that. No insurance, no operation.
The boat was preserved in a private industrial slip until 1973...steamboat enthusiasts bought it from the estate of the scrap-man. But in ripping the superstructure off, with intent to rebuild, they found that it was just cheaper to build a replica steamboat with oil firing and steel superstructure...so that's what they did, and the old hull sank at the pier, abandoned.
Long story to make a point. Probably the United States just couldn't be made compliant and livable in a way that made economic sense.