I rode a few powerbacks as a teen pax in the 70's - DC-9 or 722.
I think rising fuel prices had a role in discountenancing them (earlier engine start, and thrust burst - wasn't it Richard Branson who called for tug service all the way to the ILS hold lines to save a few pounds of fuel before engine start?).
It doesn't take a lot of fuel - but totalled over 1,000 flights a day for a big carrier, powerbacks can add up to a noticeable chunk of cash. And carbon output.
Additionally, of course, there are issues of sandblasting the ramp workers and the big plate-glass windows in today's gate lounges with debris. And that constant bugaboo - noise.
Not "politically correct," on a number of counts.
But technically workable.