The problem with refering back to past times is that speedway crowds were much bigger all around in the 1970s/early 80s, when Wembley was attracting 84,000, Brandon was getting 7,000 for a league meeting and the British Final used to fill the place. There is no real evidence to support the theory that Wembley or the Olympic Stadium would attract a bigger crowd than Cardiff in modern times.
Also the lack of a roof on either of these venues is a massive downer, as anyone watching the World Cup on Saturday night will know, imagine going to the expense of installing a temporary track and then getting a wet weekend, the financial losses would be massive !
Regards Martin