Complexity is not a problem if something is achieved with it. That said, a complex problem will often require a complex solution and sometimes the simplest solution is the most appreciated, efficient or elegant.
Poor maintainability /extensibility / modularity /scalability, bad encapsulation, abundance of side effects, spaguetti and raviolli code - among others - are perhaps the real problem you're seeing behind your coworker's complex design.
It is not a war that can be easily won though, you can see how in the java ecosystem unnecessary complexity is the norm. And outside java too we see unnecessarily complex frameworks or libraries gaining heavy traction.
Poor maintainability /extensibility / modularity /scalability, bad encapsulation, abundance of side effects, spaguetti and raviolli code - among others - are perhaps the real problem you're seeing behind your coworker's complex design.
It is not a war that can be easily won though, you can see how in the java ecosystem unnecessary complexity is the norm. And outside java too we see unnecessarily complex frameworks or libraries gaining heavy traction.