My thoughts exactly. I’m honestly of the belief that post 2000 is similar enough that all should be available unless they were one off livery’s for larger airlines (For smaller it adds diversity to their fleets) or if they were special, which is when they should be events.
Pre-2000 I think should be played by ear. If the types are ones that were seen in regular use during the 2000s and 2010s, such as the 737 classics, A320s, MD80s, 757s, 767s and F100s to name but a few, then they could also be added in for increased diversity. I think the likes of Air Europe and Inter European Airlines would fit nicely at player airports without looking out of place. 737-200’s I’m a bit on the fence about and think if they were post 2000 then that’s okay like Air One, LAN or Lithuanian Airlines but the pre-ones like above for Air Europe and Inter European maybe could make for specials.
It also means some highly requested liveries like the Thai MD-11 could be added as regular and some airlines which vanished on the cusp of the decade like Northwest (With their B742 pax aircraft which Delta retired a month before the new year in 2009, just pushing a regular American passenger B742 the wrong side of the cutoff 
(You can see the dates on my B742F post) could also be available.
I understand technical limitations but I do feel we should be able to run our airports how we want. At the end of the day, I can run a Concorde and Comet fleet out Heathrow with A21NY’s and B789’s parked next to them, so realism is a bit strained if that’s the issue. I do believe current livery’s that visit in-game airports should be priority but I’d like to have the aviation I remember available for my airports which take me back to a more nostalgic time.