"We want to keep it simple, if possible just stuff everything into one assembly. DI is an uneeded complexity with no benefit".

The benefit is its promotes designing to interfaces and allows easy mocking. Which consequently facilitates unit testing. Unit testing ensures reliable, modular and easily maintainable code. If your boss still maintains its a bad idea, well there is nothing you can do - its accepted industry best practice that they are arguing against.

Get them to try writing a unit test with a DI framework and mocking library, they will soon learn to love it.