To be clear, since the mobile app users are decreasing, what you would actually need to claim is that "web users are more likely to continue using the app than mobile users are", not just that "there are a lot of web users", since the base assumption is that web and mobile users use the site in similar ways. I don't really see any reason to believe the former, and actually having worked on social media extensively I think there's a strong chance that the opposite is true, that if mobile users are declining web users are declining even faster. There's a reason sites push mobile apps so much and it's because retention is normally far better.