With regard to traffic density, the one thing that has changed markedly in recent years is the popularity of light aviation. Microlights, for example, seem to make up a significant proportion of the new registrations published in the mags every month.
The other key issue is the "honeypot effect" around VORs, VRPs, bottlenecks between controlled airspace etc. How many people routinely plot a VFR route using convenient VRPs on the chart?
If we were to get into the habit of using other features as TPs or waypoints, then I'm sure could reduce the concentration of traffic in some areas. I routinely fly into an airfield where on weekdays the joining and leaving procedure is via one VRP to the South. You need a dozen pairs of eyes on occassion, as aircraft travelling in opposite directions, some non-radio, all converge over one small village.
Plaistows is a busy microlight strip, complete with a thriving school, yet also happens to be in an area where low level traffic will be routing around the gap between London CTR and Luton CTA. As the strip is close to a very prominent motorway junction, often used as a waypoint when routing around that way, it's almost inevitable that traffic densities there will be higher than average.
Add in the fact that en-route VFR traffic may be concentrating on checking where they are at that waypoint, and microlight traffic heading into Plaistows will be concentrating on their join and approach to the strip, and you have some of the ingredients for a terrible tragedy like this.