We have a 30+ year history that an inadequately specified "priority" field
will go unused.
Actually we have exactly the opposite. We have at least two inadequately
specified QoS mechanisms for email already (priority and precedence header
fields), and both of them are in wide use.
I suspect the point is not really that they will go *entirely* unused, but
that a poorly specified priority mechanism will not be very useful overall,
and so will not be *widely* used in comparison to the scope of Internet
mail. Stated that way, I'd have to agree.
But that's not what was stated. The statement was that it would go unused.
That's demonstrably false. We may quibble about the level of usage, but it is
certainly nonnegligable, and that sufficies to disprove the point.
The bottom line is that I think this is one of those things that the world
wants. (Needs is another matter.) So if we specify a priority mechanism that's
reasonable and implementable it will deploy rapidly and widely.