For us scaling was never an issue; We did the stateless scale-out thing. Even solving statefull is relatively easy (at least in Java) with solutions like Hazelcast. The main drawback is there are usually only a few services in your monolith that are required to scale out. But you have to deploy the entire thing when scaling out.

As for OSGi, OSGi is complex; I dare to say that the complexity of OSGi rivals that of a microservice setup. If I had a nickle for every classloader issue I debugged... ORM was especially fun (For example wrote this piece way back: http://www.datanucleus.org/products/datanucleus/jdo/osgi.htm... ). But I must admit that I don't think we could have created (and maintained) such a large modular application without OSGi. Debugging itself is also way more complex. When an issue arises you spent a lot more time tracking down which module is misbehaving. Even though we had inserted lot's of probes (which ended up in graphite) and log statements (which ended up in graylog) to counter that.

In my experience writing smaller, simpler applications (which I acknowledge also have their own complexity with distributed debugging) are still easier to understand then an modularized application.