Does your question relate to popularity as a spectator sport, participation sport, or both combined? From a participation point, some sports, especially motor sports would be less popular due to cost (and possibly the danger)
The Grudziadz match live on Premier Sports, which previously disappeared from my EPG, has now gone back on it. I believe even Dave Goddard said yesterday it would not be shown live. About time Premier Sports started updating their EPG correctly.
The 2nd/3rd tier in rugby league have abandoned the season. Can not make it cost effective if no crowds allowed in until October. Can not see that speedway can pay without crowds, and October too late to start.
Would also need to consider how many riders with temporary jobs will want to keep them until next March.
Don't believe covid testing is the relevant point. Speedway can not pay without crowds, struggling even with them. An October start would be too late for a meaningful season.
The post you refer to suggests only the top 50 in the world. If there was such a list, how many of them would be riding in GB. Possibly only Doyle Pedersen Lambert?