When the monolith goes down, it's hundreds of thousands, sometimes millions of dollars of revenue lost both during and as a result of an outage.
You really, really don't want a problem with eg internal staff vacation hours to do that.
It sounds like you're describing a monolith that actually contained multiple completely independent applications - which I don't think anyone would disagree with being a good case for splitting.
In most cases I've seen, the decision to split into microservices usually involves a fairly high amount of dependency between different services to achieve a common business goal (hence the concerns about things like referential integrity)