Grails application is a brick once it is built. It is really hard to add anything to it at runtime.
Lots of applications that go beyond 15 minutes blog demo need some sort of extensibility platform. Examples of such:
a) Multi-tenant SaaS deployment - adding per tenant extensions that would require redeploy the whole application is a pain
b) Vendor made enterprise apps - once released these applications need to be enhanced by the customer application group. Grails doesn't have a good answer for this either - other than shipping the source code.
c) Large applications - the build time grows as application grows. Modularizing larger application would improve productivity. Binary plugins sort of answer this problem but I feel like if there was a way to build and deploy a piece of the larger app at lower cost it would be better than recreating the whole thing.
IMO OSGi as technology is dead is a bold statement so as the statement about increased complexity. In fact you guys @Grails decided to break your code in modules and seemingly increased the complexity of the whole thing.
Hopefully you'll reconsider one day.