Blizzard and the Warcraft Team has pushed 20 Man Mythic as the "Premier" Raiding size for nearly the last 8 years. Since that point Mythic has slowly declined in popularity and participation has rapidly decreased in Shadowlands alone. Just this last tier alone encounters had to be nerfed several times in order for the 1% of guilds to clear a boss in the middle of the raid, a heroic boss was nerfed 4 times the first week of the raid being released, and top echelon guilds have opted out of re-killing the last boss of the tier because of a bug being fixed and making the encounter significantly more difficult. 

10 Man Mythic should be the new Premier Raid size moving forward. This opens up players who want to raid and don't want to deal with multiple cliques with conflicting and clashing personalities that 20 Man Raiding fosters. This would enable players to have more flexibility to do high end content with friends without having to compromise by recruiting extras to make it to 20. Shifting down raid sizes also wouldn't be harmful to the current guilds as they could either shift into 2 raid teams or have more competitive rosters.

Back during Cataclysm and Mists of Pandaria 10 man raiding was more popular than 25 man raiding, with more guilds clearing raids as 10 Man Heroic vs 25 Man Heroic. For example in Siege of Orgrimmar, 2792 10 Man Guilds cleared Heroic vs 1026 25 Man Guilds cleared Heroic. 

More Data showing that 10 man was as popular or more popular depending on when: https://drive.google.com/file/d/1h2jgBTO4myXj8lF6bkQSaOI2BZRxH9p_/view