Big aeroplane supersonic = big sonic boom = overwater (or desert) flights only.
Big aeroplane which cruises at >M1.5 not efficient <M1.0
Same aeroplane = megabucks.
Market too small to make business case.
(If you made transpacific the market it may work, except that you've now emptied the premium cabins of the transpacific a/c which is where the airlines make the money)
To date, any engine which works at high speed WILL be noisy at low speed (departure/arrival). Chapter 3 marginal, no hope of meeting regs in force at time of entry into service.
If there is ever a Conc replacement it will almost certainly be a biz-jet. 'til that time they're likely to remain king of the castle.
