Sounds like elitist crap to me. The very sort of thing that once put many off even trying running. If you want an elitist marathon based on time objectives from previous marathons get a Boston time.
It also elevates London to some mythical marker for marathon running when in fact it's just another marathon. It's so popular as it's broadcast on the BBC. In the same way do you think the Great North Run would attract half the numbers it does if it wasn't broadcast live every year? Simple rule broadcast things like this live every year and they will attract a greater volume of participants, veteran runners or newbies.
Let's face it are the veteran runners desperate to run it as it's the one folk have heard of because it's on the telly in any position to look down on the newbies that want to run it because they saw it on the telly? I think not. Plus, for every didn't train or even turn up for it there will be someone who ran their first of many marathons and caught the running bug.