+1 to this. Especially with go, most of the (non-integration) tests I end up writing are behaviour driven tests. I'm not really sure how one can write these sort of tests without interfaces and mocks.
One could use build tags to provide alternative implementations over a concrete type, I suppose.
But like with everything in life, there is no right solution. Just different tradeoffs. If interfaces best align with the trades you are willing to make, go for it.
