Your question is a bit wide, so probably that's why noone bothered to answer yet. Anyway I'll give it a try and show there is no definite answer
707; In the 1960s it was the flagship main intercontinental jet for say Pan Am, TWA, Air France, BOAC (BA), Lufthansa etc.
All bought 747s for that role in the early 1970s. 707s soldiered on with main airlines til the mid 1980s in secundairy roles. L-1011 Tristars replaced the last 707s at Pan Am and BA
, while PIA, Air India, Air France, TWA, Lufthansa etc took A-310s and 767s. But airlines which in the end used them mainly domestically like American, TWA also took more 727-200s in the early 1980s, which were the same size.
An exception, MEA
which continued flying 707s til 1998, replaced them by A-320s and 321s, skipping the 2nd generation of aircraft.
721; big original 721 operators like American, Lufthansa, Pan Am, Eastern and United took MD
-80s, 722s, 732s and 733s in the late 70s and 80s to replace them.
DC-8; same story as 707. The flagship role with KLM, UAL, SAS, JAL was taken over from 1970 onwards by the 747. Later came the DC-10s, 767s and A-310s to replace the last survivors in the mid 1980s on secundairy routes.
DC-9; the major operators bought MD
-80s, 737-300s, Fokker 100s and A-320s to replace them from the mid 80s onwards. Northwest have hung on to them long enough but partially replaced them by A-319s and the rest might be replaced with Embraer and CRJ jets.
Convair 880/990; a small fleet anyway, American, Delta, Swissair and TWA replaced them with 727s and DC-9-50s
-10; only BOAC/BA and Gulf Air were operators to write home about. Was the secundairy long haul aircraft in their fleet from the start. Even some were replaced by the 707 because it had lower RASMs. Later the survivors by L-1011s.
Trident; only BEA/BA and CAAC were major operators, both took 737-200s and 757s since the late 1970s and 1980s.
Caravelle; the original airlines took DC-9s and 737-200s from the mid 1960s til the late 1970s to replace them, and 727-200 for the bigger routes. Air Inter and Syrian kept Caravelles til the early 1990s by which time they could skip a generation and replace them with A-320s. Nowadays the A-320 is what the Caravelle was in the 1960s; almost all European airlines except KLM and Olympic fly them on their European routes.
I hope this is of some help, I always love talking on and on about 1960s-80s airliners.