Microservices aren't about scaling distributed systems, it's about breaking every little thing into it's own service. In fact, it's not uncommon to have all your microservices run on the same machine.

Obviously the exact mechanism you use for your micro- model will vary, but the concept still lines up in breaking things into small pieces. In a microkernel, you replace function calls with IPC, for a microservice, it's RPC, and for your 'microuserspace', perhaps a syscall-esque API to the service?

Though reading your 'microuserspace' concept again, it may line up more with FaaS, which is certainly hot in the webdev world right now. Though the purpose of the two would likely differ.