Note - I don't agree that past IDs should be posted after expiration
without the author's consent. They were submitted with that
understanding, and post-facto changing it by the IETF is not appropriate.
I would rephrase the above as whether it is appropriate to take
someone's work and post it as your own or use the work. For the former,
I don't think it is appropriate. For the latter, the author has given
the IETF change control over the work. I'll ignore the exceptions.
That does not mean that the author should not be credited for the work.
If I am not mistaken it is IETF practice to do so.
Actually regarding reusing the former drafts we have been discussing
this in 2008 at depth when one draft reused most of the text of another
draft.
The end result was that technically and legally it is appropriate.
Reference section 3.3 of BCP78 / RFC5378.
However while the above documents do not mandate it I very much agree
that it is in good taste to first ping the original authors about your
intentions to extend and build on top of their idea.
R.