Is multithreading a complex subject that should be handled only by experts?

This is a fundamental question, that deserve proper care. If experts are mandatory, the problem is that they are hard to get by. Assuming you have enougth of those, we need to address the problem of code integration with non expert developers. Having the expert turning non multithreaded code to multithreaded one does not work; actually it can work at the algorithm level, but at the application level. And then, non experts can no longer modify the code.
The other way around would be having experts lay down some scaffolding code for the rest of the development force to fill in. It looks like a better approach because fix can be made by anyone, as long as it does not pertain to the scaffolding code.
Let’s go one step further and isolate scaffolding code from so called ‘business code’.
Et Voilà!, we have a nice framework. Which has been designed by experts from the ground up.
That is in essence my opinion: all developpers must be able to leverage multithreading. But like several others technical fields, such as databases, middlewares or 3D to name a few, we need to remove the burden from the mass of developpers.

The next question is then: how do we design such a framewok to be a pit of success.