Is the microservices architecture a must for updating web apps with 0 downtime?

Suppose a solo developer making a platform, of which the core feature is messaging between its users. The developer already made such platform which works without any hiccups. Now, when he thinks of publishing it, he realizes that he will need to update to fix bugs, add new features, etc… There is no way he can do Blue/Green deployment cause how can user 1 in Blue communicate with user 2 in Green? The only way is if all the data were to be stored in some shared database. Is his only option to switch over micro services?