I don't want to get controversial - but I will.
I think a big part of Speedway's problems started with the Laydown Engines, and the 4-Valve bikes used in the 70's. It started the free for all with the Engine Tuners. Spiralling costs due to Engine Tuning etc. contributed to increasing the price at the gate, thus taking the Sport out of reach of very many people. Costs became prohibitive crowds grew smaller and smaller. This is where we find ourselves now.
There is no doubt in my mind that the old 2-Valve was cheaper to run, safer for the riders, and no real advancement has been shown on the Track. The 2-Valve may have been fractions slower but, what we gained in speed, we also gained in injuries. Tracks built for Speedway nowadays can't cope with the the new 'rocket ships' that riders are using at the moment, hence the big increase in injuries.
Tracks haven't changed much since 1928 - but the bikes have. THAT to me is a problem also.
I don't believe that the GP System has done British Speedway any good either. Personally, I much preferred the old 'one off' World Final.