zonoma,
ATC does not keep aircraft at SID altitudes, to increase fuel consumption. They do it because there are other aeroplanes above or below, which are in the way. The same applies to direct routings.
A year ago, Big Airways demonstrated 'The Perfect Flight', EGLL to EGPH. Unfortunately it was demonstrated about 1700 UTC on Saturday, not at 0700 UTC on a Tuesday, when everyone else is trying to achieve their ideal vertical and horizontal profiles.
Sadly, until there is a strategy to spread the load over 24 hours, then, just like the roads and the railways, there will be delays.
Every system, unfortunately, has its own 'carrying capacity'.