This would presuppose only a single new version being "in-flight". But a microservice change often bleeds into another microservice having to change. Multiply this by all the agents working on the product, and you get a very complicated release process for those microservices.

The ripple effects are basically the same as what you'd get in a monolithic codebase. In fact you can still think of a set of microservices as a single codebase, just not centrally maintained anymore. The complexity is moved rather than eliminated. And you'll still have agents (and people) stepping on each other if there's too little coordination.