That isn't enough. If MS thinks 1 new AAA title a month is enough then this service isn't going to be as successful as they think. Niche stuff like Scarlet Nexus and failed games like Avengers isn't getting people to sign up in droves.
You say "those games make it worth it" but do they? It might be enough to keep what they have, but there is a reason why day 1 games still sell so well. Only a small portion of the consumerbase like either Horizon or Halo, that portion shrink massively for those who like both. Plus, Halo's MP is F2P anyway.
Simple fact is they aren't offering enough. 18-20 mil subs is only a third of the X1 consumer base, or a fifth of the 360 consumer base, and this service is on 2 gens of Xbox, and PC, and mobile.