That's fair, and that's your preference; my preference is the opposite - that when a type can be consolidated in to a single, 4-line declaration, without any extensions at all, it actually becomes easier to understand.
But again, these are our individual preferences. Hopefully we can agree that both are reasonable, and that it would be regrettable if the language favoured one way of organising things.
My remarks are not necessarily targeted at the
conformance keyword as proposed; but at the idea that extensions already give us all we need. Since this is a "pre-pitch", I'm trying to elaborate on some of the potential use-cases so we can get a fuller understanding of the problem developers face.
Grouping things in to extensions is also an optional strategy for enabling these diagnostics, and as discussed, in some situations it can be impossible as the language is today, and in other cases, even when it is possible, it can be non-obvious.
We tend to say that writing conformances in extensions is more idiomatic, and for complex types and conformances I would agree, but it is not always the case. I hope we don't start using that as justification to disregard other, supported ways of doing things.