If ocean liners cost significantly less than flight and only took twice as long I think they would be quite popular indeed. However the real trade-off is not comparable to Concorde.
Ocean liners did cost significantly less than airliners in the beginning. A transatlantic ocean liner ticket in 1950 cost about $1200 in 2020 dollars while a one way flight to europe in 1950 cost around $3000 2020 dollars.
Taking a 5 day longer trip to save $1800 is equivalent to getting paid $360 per day. If your salary is less than $90,000 per year, you'd be better off taking the ocean liner.
And in the 1950s--at least before the very end of the decade--you're talking prop planes likely with fueling stops in Gander and/or Shannon. So transatlantic air travel was probably not the most comfortable experience.
Taking a 5 day longer trip to save $1800 is equivalent to getting paid $360 per day. If your salary is less than $90,000 per year, you'd be better off taking the ocean liner.