It's actually a fine solution and I often do this with infrastructures that I don't intend to maintain myself. In general, however, I try to minimize complexity

It's actually a fine solution and I often do this with infrastructures that I don't intend to maintain myself. In general, however, I try to minimize complexity, and having an external service feels a tad more complex than running the database inside the instance. But you could probably make a successful case to the contrary - namely, that using a service rather than running your own is actually much simpler and easier to maintain. I must confess that in this decision, the complexity of the AWS console (which I consider awash in buckets of accidental complexity) usually tilts my balance towards self-hosting.