"You can write shit, unmaintainable code in anything, but to point at pjax and granular key-based caching schemes as somehow specifically prone to this? What? That doesn't make any sense to me."

To take things to the extreme, if you chose to write Basecamp Next in pure x86 assembly (for speed, of course), I think we'd all agree that the code would be much more prone to maintainability problems. Similarly, it is plausible that choosing to write Basecamp Next entirely with pjax+backend MVC+granular key-based caching could be more prone to maintainability problems. Not that I have any particular reason to believe it will, but we're wondering if there are any particular reasons you believe it won't?