They are not forgotten as many are being used around my area. I suppose the worst of those bikes was the Douglas which was well built but needed to be a 500cc bike.
On the Dartmoor ride this year, there were no Velocettes, Ariel leaders or Arrows, Douglas, B50s, TR5T, Norton Mercurys, Commandos, Gold Stars and quite a few more of the large cc bikes and that is because older riders cannot kick start the big ones.
The ride had a lot of bikes but so many are new ones and many of the previous years riders are no longer riding.
People who do ride old British bikes are generally very familiar with the older bikes that used to be common.
20 years ago there were Manx Nortons, Velocette Thruxtons, Gold Stars, Rudges, Calthorpes and quite a few 1930s bikes but there are none there now.