2.4ghz should be strongly encouraged yes, but there are some VERY good 40mhz systems that i wouldnt swap for 2.4ghz (any seen a Futaba 14MZ? ). And i dont believe the reported problems are caused by the 2.4ghz radios.
Its worth doing some tests to see what the cause is, but it isnt uncommon for these problems with 40mhz in general.

The point about our events being smaller does not mean these things wont happen, if anything it will make them worse.
We have never had a problem at our events, and had 20 robots in our arena fighting, and all the robots are far closer together........