They have gotten the desired life out of the airframe though. The 747 lasted a lot longer than some of it's rivals such as the L1011 or some of the McD range. Same with the mk3's, they've lived a long life and now it's time for the razor blade factory.
Sure new models that are far more efficient save money and make sense economically but passengers also expect a certain level of service, and no matter how many times you refurb a 747 or an mk3 it's still clearly the same old dated junk underneath. I know I'm not the only person who avoids airlines flying 747-400's on longhaul routes, and I suspect there's quite a few who avoid the railway due to the mk3's.
In western European countries the trains rarely make it as long as ours do before scrapping, most of the NS stock in the Netherlands dates from the 1990's or later as they realise you can only refurb up to a certain point (ok the DDM1's are 1980 but they're going very soon and were only brought back to to shortage of decent stock).
Well by your logic all the current 777’s in service should be scrapped including those that have recently been delivered as they are “outdated junk” underneath as the 777X is on the horizon which will be at the most modern standards. Thousands and thousands of 737NG’s in the air that are “outdated junk” yet its the most popular aircraft in the sky at the moment. 379’s are technically “outdated junk” now and they are only 5 years old. I am looking forward to the new stock but then again most of the current stock has served its useful life and is now very tired. But to call something “outdated junk” just because it isn’t brand new is a bit ridiculous. BTW the 747-400 has a lot of very modern avionics onboard, it is RVSM authorised. The ETOPS ratings of the 787 and A350 sealed its fate as a twin engined aircraft it cheaper to operate and maintain than four engines.