This doesn't make sense to me. YAGNI doesn't really relate to product management. When a PM talks about future extensions of the product, they're literally stat

This doesn't make sense to me. YAGNI doesn't really relate to product management. When a PM talks about future extensions of the product, they're literally stating that "we're gonna need it". You can disagree with that, and argue why you won't need that feature, but that disagreement is separate from YAGNI. In short, you shouldn't refer the PM to YAGNI (since it's completely irrelevant) and instead discuss your ideas as to why those features are not necessary. What helps me the most is to point out to the PM that software development is done better when engineer has an understanding of the whole system. So, I tend to ask PM to spell out the entire system as they desire, instead of spelling out the MVP and future extensions separately. You can first discuss what's the end-goal with the product, and then discuss what belongs in MVP, or earlier versions of the product.