LPR is not a standard, but it is widely available and both clients and
servers are implemented on every platform in existence. The
alternatives you mention are platform-specific. LPR is about the only
print protocol for which clients and servers are widely available for
almost every platform in existence.
If the WG feels that it's useful to show how to implement PSERVER,
RPRINTER, or SMB print protocols in terms of IPP, I'm sure IESG would
be happy to have those documents published as Informational RFCs.
There's no intent to "saddle IPP with LPR". My idea is that the WG
should show how to implement LPR functionality on top of IPP (i.e. how
to translate LPR to IPP). I agree that it wouldn't make any sense to
try to implement all of the IPP functionality over LPR, and there's no
requirement that this group do so.
Keith