Have you not just proved their point to some extent? If clubs are spending beyond their means to sign better riders, and presumably winning sides, but still not making a profit, then maybe not as many people are "entertained" by success as you think.
Ultimately, 2 or maybe 3 teams will get any success in a given season, from 15(?) clubs. If success is what keeps people coming back then the majority will struggle.
I'm not actually even sure great racing (or lack of it) is the real issue. I'm sure most people would rather have it than not, but for me it's the lack of competitiveness or meaning to most matches.
When you look at Ekstraliga 2 in Poland, they're all aiming and building for promotion, to get more fans/tickets sold/TV money, etc. It means something. It's like the Championship in football.
Where is anything like that in British speedway? It doesn't really matter if you finish top or bottom of the Championship, nobody will remember in 3 months anyway, you can just start again next season.