An SRE might help you build systems, but that is a lot of system to build depending on number of microservices. Managing the complexity of N services comes at a cost, maybe that cost is in how seamless and wonderful the testing infra is.
A very well built monolith is very easy to manage.
Most product devs want to trivially build a feature, not deal with the complexities of running a service in production. Abstracting away a request handler is going to be an easier overall system than abstracting services.
As for the oncall/onboarding etc, SRE is there to support and enable, not to be an ops monkey, so that stuff scales with number of services/engineers.
