Your missing a few takes on this, let me help you out.
First off GoW 1 and 2 were on a 150mil+ userbase, thats just to point out your userbase arguement.
Second, BotW sales are around 7mil (by end of 2017) and will be around 8mil after its been on the market for a year on switch alone. (Add 1.5mil for wiiu anytime later) and thats on a userbase of less than 20mil. Its an evergreen title that will sell as more and nore people buy the switch. GoW will not come close in legs.
Lastly Best selling LoZ as you said are 7.6mil and 8.5mil (TP on wii/gc) but BotW is an everchanging game to the franchise. Not to knock down GoW but they arent comparable in that reguard.
That said its just BotW though, GoW can outsell almost every LoZ ever aside from BotW