the '90s improvement can be explained by full development of professionalism & a full complement of african nations - kenya/ethiopia/north africa/etc participating fully in the sport
with professionalism + such numbers of quality athletes, it was inevitable you were going to get significant improvements in time ( even in the 800, where the record only improved by 0.6s in the decade, the number of 1'42 guys in the 90's was much higher than that of the '80s )
previous historical data is virtually irrelevant as there was a world war which ruined most of '50s & there were no significant african participation until '70s ( & that was just small number of kenyans & even fewer thopys )
the stagnation in the last decade i'd put down to the eventual impact of 3 global finals/4y cycle stifling "freewheeling" record chasing 3 out of every 4y - this year is first in 3y without a champs & we've already had a 1'42-flat, 3'49, 12'53 & 26'59 & we've hardly started the season !
all the distance track records are now solid & personally, feel have reached asymptotic levels - the progression there has been haphazard due to historical/participatory & frequency of global competition factors, but it looks there now