As with the once great railroads of the USA
Ocean liners made their money from freight/cargo/shipping including mail. The great companies like Cunard also made vast sums from carrying "steerage" passengers from Europe to North America and other places. Even late as post war period right through the 1960's for both travel and emigration purposes people came from Europe and elsewhere to the USA via ship. Just look at how full the Andrea Doria was and with what types of passengers when she was hit.
Happily for many of us this moving house by ship meant many persons brought all sorts of things to the USA including large appliances. Today with nearly everything going by air shipping large and or heavy objects is *VERY* expensive. Most only do it now if someone else is picking up the tab (military, company, diplomatic corps, etc..).
The arrival of jet air travel and as airliners built up a safety record killed transatlantic ocean liner travel at least far as passengers, mail and some cargo was concerned. One of the largest airlines in the world is owned by UPS whose planes fly all over the world with nothing but freight cargo.
Cunard kept things going for awhile with the QEII and other ships that stuck to the ocean liner class, today the newest ships are all of the Vista cruise ship class.
https://en.wikipedia.org/wiki/Vista-class_cruise_ship
An new ship built along the ocean liner lines of the past probably would not generate enough traffic after the novelty wore off to pay bills. I mean if there was a new "S.S. United States" during the Downton Abbey craze it might have resulted in good bookings. Sadly otherwise think the market for scheduled transatlantic crossings is just to small for anyone to bother with.
If such a ship ran on schedule and kept to it, then perhaps it could win some freight business which would support passenger service.
