What makes her "no longer fit for the purpose" from a pax perspective? The 787 is even noisier on the inside, in my impression.
The market has grown. You don’t have 380 PAX going JFK-LHR and then 50 connect to Rome and 80 connect to Paris and 70 go to FRA and 50 to Stockholm and 70 to Madrid. The airlines today (well, last month) can get 270 people who want to go to each of those destinations direct from JFK. Same out of any major hub. Plus the operating cost of 4 engines v. 2.