A lot of music that is produced for the charts is considered way more disposable now then it was in the times mentioned at the start of this thread. Yes that's a very general statement but the majority of the music industry is based on quick results (as much money in the shortest amount of time). In years gone by, a label would invest in a band/act and help them develop, now acts get dropped all the time. Although production techniques have significantly developed, a lot of music is far simpler and less "musical" (in a traditional sense). I think another factor in this is originality, I think originality is very close to it's end (as depressing as that sounds) so when these songs in the 50s/onwards came out, they genuinely were original and in some cases ground breaking where as that effect is rare now. I guess only time knows what songs will be around in the next 10 years, let alone the next 50 to 100 years...