gui design - 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.

Answer

Only when a change would be absolutely earth shattering. Changing the standard keyboard layout would mean hundreds of millions of people can't type any more. It's a change that affects the majority of keyboard users the world over at this point.

It might seem like this is a good reason to not change things when the change is disruptive, but it's really not. Most changes people can get used to over minutes or days. The problem with keyboards is that these are physical devices that can not feasibly be replaced. If you change keyboards, hundreds of millions of the old ones are still out there. There's no reason to change. You can't force the change.

In the digital space, change is almost always possible; Windows Vista made a great deal of people freak out, and the Word 2007 Ribbon confuses new users to this day, but you can adapt to them (especially due to the significant UI improvements in Windows 7 and Word 2010).

If you can force a change, people will get over it. You better have a good reason to force the change, as there will be short-term consequences, but if research really does indicates performance improvements, the change should be made. Allegedly Microsoft did find significant evidence supporting the Ribbon, hence their change.

The reason you really need to force a change is the longer a problem remains, the bigger a problem it becomes. Changing keyboards would have been easier in the 70s. If computers used different keyboards than typewriters it would have been a little annoying at first, but keyboard penetration was small. Now the level of inertia is almost completely insurmountable.

Change Fast when research supports it. The digital space has a lot more room for change than the physical one for many reasons, which is why you need to be sure physical things work before you distribute them.