It is arbitrary. But there should be cutoff somewhere. Heavier vehicles are more dangerous (and tend to have lower safety standards). So there should be more licensing restrictions on heavier vehicles.Sounds good, however I would be concerned about the weight? Maybe it is time that the government increased the weight that can be driven on a normal licence? Never understood where the 3500 came from.
As it stands, even if the limit were increased to 4.25t for EVs, it would be as restrictive to motorhomes as the 3.5t limit for diesels. So electric motorhomes at 4.25t would probably be very payload limited beyond 6m.