I'm not sure what the answer to the question is, but the old spotter in me yearns for the days before my local airport, Newcastle, became an endless succession of 737's and A320-series.
In my very early years with my dad on the roof at NCL we saw DC-8's, DC-9's, DC-10's, F27's, F28's, 707/720's, 727's, 737's, 747's, BAC 1-11's, Viscounts, 748's, Caravelles, Tu134's, Tu154's... and I've probably missed a few off that list.