Michael wrote:

But, how much longer will it be before /G is a de facto requirement?

IMO, more than 5 years but less than 15.

Already when I fly IFR (filed /U) controllers give me instructions
("proceed direct foobar") that require GPS

Well, they don't really. I bet you can do that with the M1 LORAN. Or
you could if it didn't come with a placard limiting it to VFR use only.
A handheld GPS will not come with such a placard, and there's no rule
that says you can't use it for enroute IFR (anyone who says otherwise
is welcome to quote chapter and verse from the approriate regulation -
NOT an advisory circular or AIM).

Try 91.205 (d) (2) for starters:

d) Instrument flight rules. For IFR flight, the following instruments and
equipment are required:

(2) Two-way radio communications system and navigational equipment
appropriate to the ground facilities to be used.

Think non-radar operations, where the controller isn't going to play "Frick
and Frack" direct-to games with you. Failure to comply with 91.205 can
rapidly lead to 91.3, and the FAA attorneys win every time.