I don't think you can write maintainable code without monads. Code has to be written in the language of the business domain so that it can be understood in those terms, so you can't (to take an extreme example) have every other line being "if err != nil ...". But if you leave secondary concerns completely implicit then your code also becomes unmaintainable because you can never understand what any given function is doing (at the implementation level) and so you can never maintain anything. You need the dual perspective that monads or something like them give you where you can say "this is a secondary concern in this code but it is there".

It's not about "programming cleverness" - there's nothing "clever" about monads, rather the monad is a very simple and boring interface that turns out to be a good tool for helping you write code that does what it looks like.