> The problem I've seen when yagni is applied is not building something now leads to a costly data migration down the road when you actually do need it.

I see YAGNI fail when it's used an excuse to not think about the future at all, rather than a reason to delay building parts of a system that aren't needed yet. Relative to building software, thinking about it is cheap.

I usually think about:

    * how needs might change in the future
    * what would have to change in the current system 
      in order to satisfy those future needs
    * how difficult changing the system to meet 
      those future needs would be.

Ultimately, I know whether the current software design can easily adapt in the future.