Well, the subs are not really high compared to how the number of pc players grew in the past. When WoW was at its peak, 2008, there were around 1.1bn pc gamers. Today it is 1.86bn PC gamers. Source:
https://explodingtopics.com/blog/pc-gaming-stats
While wow had a total concurrent sub count of 12-13m subs in those days, it nowadays has 8m subs, shared among multiple different versions of world of warcraft. While it had the same amount of subs earlier for retail.
Compared to the grown size of the total PC playerbase, and taking in credit that way more than a total of 100m players already played WoW, it obviously cannot adress any kind of majority of players for any longer. Also considering that the amount of players drops always short after a new expac or classic version releases.
In total: WoW changed from being the first broad audience MMORPG to adress different niches. WoW retail nowadays adresses transmog casuals and hardcore raiders / hard dungeon players, classic adresses old players from the former generation, hardcore addresses a small hardcore gamer niche and a large youtube video audience. And whenever retail releases a new expac, it lures in millions of players just to drop down back to a new low after either more or less of expac lifetime. Infact, blizzard abandoned the mass market in favor of a few niches, while they could adress way more players than in 2008 nowadays, and even could gain back all the old players if they either would adress their gameplay needs in additional versions of the game or if retail would cater to many again.
That is at least my conclusion. You are free to disagree.