I don't think you can stop less experienced engineers (or experienced but still bad engineers) from contributing to projects - especially at the scale of a lot of these large organisations.
After all, for people early in their careers, being bad is the first step to being good and it's impractical (or impossible) for organisations to exclusively hire "premium" engineers.
There's also nothing wrong with engineers who program exclusively to collect a paycheck and aren't actually passionate about their work. I believe this is an inevitable side effect of the accessibility of programming today.
Project architects should instead strive to design their systems such that they expect low quality contributions and limit the potential damage those contributions can have. Systems should be able to take a low quality contributions and if the need arises, have the capacity for those circuits to be optimised at a later date without rewriting the entire thing.
I personally find the argument that "bad engineers make microservices bad" to be silly. Bad engineers would poison any poorly architected project, be it a monolith, microservice or otherwise. Microservices are just an architectural optimisation with trade offs that some organisations find value in.