Let's break it down. CompuServe, the oldest of the big 3 dialup services,
never had more than 250K subscribers, and it was bought by AOL in 1997. When AOL went public in 1992,
it had fewer than 200K subscribers.[1] At its
peak, in 2002, AOL had 25M subscribers.[2] The rest of the dialup services never totaled more than 500K subscribers combined, total for their entire existance.
Again, in 1990, the Internet had 350K hosts worldwide and over a million users. The total number of dialup subscribers in 1990 doesn't begin to approach that number, and it doesn't matter if you pile on all BBS users, because BBS at its peak, low hundreds of them. Within 2 years of WWW arriving, there were only 20 left.[3]
So take your 10K BBS users, take all the CompuServe users ever, 250K, take the 200K AOL users in 1990, and all the other dialup subscribers for the entirety of their existence, 500K, and with this screwed up cheating math, you might get a million to match the over a million Internet users in 1990.
[1] https://time.com/3857628/aol-1985-history/
[2] https://upload.wikimedia.org/wikipedia/commons/6/61/Aol_subs...
[3] https://www.theatlantic.com/technology/archive/2016/11/the-l...