[jrochkind1]

Absolutely.

But it's really hard to tell when it is and isn't reasonable, when it comes to UX.

I find that the only thing you can do is try to have peopel who aren't developers and don't even know what's "easy to build incrementally on top of the existing stack" determining the appropriate UX for the customer/market/business need.

Then the developers can say "OK, but if we did it like THIS, it would be a lot cheaper to implement because we can incrementally add to what we've got", and the product manager/owner can push back "Eh, that's not going to meet the market need", "OK, but we can't afford it" -- it's a negotiation. But in my experience you really have to have someone who isn't a developer at all standing up for the user need, for anyone who will be involved in the implementation, no matter how smart and user-centered, it's just too tempting to decide that the thing that can be met with the incremental change to existing stack is "nearly as good" no matter what, when it's so much easier and more elegant under the hood.