I don’t know about subs but the amount of people in BGs has decreased to a point where you play with and against the same people a lot more often. I sometimes run into guildies even if we haven’t joined queue together and I face the same guilds quite often (BG Besties, Beekeepers etc.) as well.
Everyone has different reasons why they do BGs less (if they do BGs less that is), to me the current iteration of PvP is the worst I can remember. Everyone has a huge amount of procs, the amount of CC and pseudo CC is just stupid but at the same time it is needed because healer healing output is even more stupid. I don’t blame people not enjoying this.
Like you said, the three syncers would barely be noticeable if there was a larger pool of players, which it felt like there was even in Shadowlands.