I can only see this getting tighter too and the "we're running late, any directs please?" being met with a stern "no". Why? Say 20 aircraft get airborne from various Spanish airfields and are expected to arrive exactly 1 minute in trail when they get to the UK entry fix. The first couple have filed a TAS of T460, but are actually only going along at M0.76 as the company have told them to only fly at IAS245 at all times. The next few all fly at M0.77/78 along their standard route and get their optimal requested flight levels. The last few are stuck underneath the ones already airborne and only get FL310/FL330, but they do get some great directs saving up to 30nm (Spanish boundary direct to the UK entry point). On top of this they are also a little late so they go full pelt at M0.78/M0.79. Now you are faced with a situation at that UK entry point where the 20 aircraft 1 minute in trail are now 20 aircraft crammed into 20nms from first to last, all wanting to get into the same airport, and the sector in question suddenly gets an unexpected overload that flow control could never protect.
There is a route through the airspace I control that the "usual direct" can save an average of 6 minutes on the flight time in UK airspace alone, that can be enough to overload a sector 1000nm away.
TheiC you ask why can there not be more flex in the system - simple, compatible & dynamic systems. For us to climb an aircraft to a level other than that filed means we would need to assess the impact of doing so, and making sure if that pushes a sector such as one of the Marseilles ones over the edge, that everyone in the line from me to Marseilles know that this aircraft needs to be level changed to go through the original planned sector. Systems just do not have that capability and flow control at this time isn't that dynamic.
I wish it all could be far more predictable because the consequences aren't that great.