I think there is definitely a market for streaming at the right price...
One of Speedways weaknesses also can be it's strength...
Due to the team model it uses, you don't have the same tribalism that other team sports can engender, however the sport does have a large proportion of followers that generically follow the sport, and very closely..
Meaning any meeting, regardless of the two teams involved, will attract their attention..
I watch Swedish and Polish meetings on Tuesdays, Fridays, Saturdays and Sundays, but won't lay out £14 or so for a UK stream..
Make it £7-£10 and I probably would, at least once a week...
In the Speedway Star, Gerry Facenna mentions how surprised he was by how many fans from overseas have purchased their streams..
It could be a big market if the price point is set well, and clubs don't just think that the fans of both teams are their target market, (or might not attend and buy the stream instead)..
Glasgow say they only streamed due to crowd increases in the stadium, and, once again, are using their own fans to help drive bigger crowds with free tickets..
That's the key...
If a promoter thinks "I won't stream as my fans will stay at home and watch", then they either are not promoting well enough, not providing enough entertainment, or the competitions they run are not engaging enough people...
Do their day job properly, and streaming should be the icing on the cake..