Good reasons to use bad UI

When should we make a conscious decision to retain poor UI, even if temporarily? Not because of budget or time constraints but because of what I call the QWERTY paradox.

In short, the QWERTY paradox is that users have a better experience with a poor interface than with any of its contenders because it's been so widely learned.

QWERTY Paradox: The letters on a QWERTY keyboard were originally arranged to prevent typewriter typepresses from jamming. Of course, this is a not an issue today and the arrangement of the letters are actually rather inefficient.

There are several more efficient layouts for keyboards which have been around for a while such as the Dvorak keyboard. However, a good computer designer with a passion for usability ought to ship a keyboard with a QWERTY rather than a Dvorak keyboard.

The former has been used so often, users are much more efficient with the convention of QWERTY than with the unfamiliarity with Dvorak.

So my question is, On what criteria should we opt to retain bad UI to keep our UX pleasant (paradoxically)?

UPDATE

I think the point of the QWERTY paradox may have been lost. Essentially it's this:

The gain from an improvement can be significantly less than the friction from migrating to it.