Good point with the longer buildups and breakdowns in the older dance music = actual longer mixes.
The attention span of a clubber has diminished I think - it's all about the 5-minute edited peak, trough, and peak, whereas the older style tracks were longer and had very DJ-friendly intros and outtros, not to mention a middle portion of simpleness which also permitted an earlier window to mix out of.
You only have to look at Mr Armin van Buuren's early stuff, some of them are 12-minute wonders! You'd never see that today.