> What you wrote suggests the data will bias predictably

Ex post facto. Before the fact, we don’t know.

Imagine you know the weather will be a strong gust regardless of direction. Averaging the models will produce a central estimate. But you know it will be biased away from the center. You just don’t know, until it happens, in which direction.

> They can provide for the continuation of data visibility/granularity by releasing the prior numbers as previously calculated and at the same time changing the calculation of the headline number to be better compensated

They do. These data are all recalculated with each methodological change. They’re just deprecated indices the media don’t report on because they’re of academic, not broad, concern.

> simpler argument is that changing it at all will result in a negative step change in the reporting

Simpler but wrong. Those data would be useless for the same reason we don’t let CEOs smooth revenues.