Thanks for the lengthy, thorough response

A badly located tree is never going to thrive. It could struggle on for
quite a while, attracting pests and requiring extra attention. It might
be best to pull the plug.

It has fruited in years past, but that doesn't mean it will.

A related question - were I to plant a new fruit tree in its place,
would I be wise to stay away from apple (or pear?), so as to not give
the aphids a new similar food source?

How long is the crawl over?

from tree to tree? Perhaps a yard. This is a San Francisco backyard,
after all

Is the runty tree upwind of the others?

yup

The Organic Method Primer UPDATE suggests gently digging around
the base of the tree looking for aphid galls on the roots, which can be
pruned away and destroyed. It also suggests using beneficial nematodes
to attack any colonies on the roots.

Ah - good suggestion. Probably not a backyard chore while my 3 year
old is around. What nematodes does it suggest.

And wht is the OMPU $200 on Amazon?!?

According to what I just read, they have an alternate host on elm trees, which
they feed on in the early spring for two generations before moving back
to apple trees. (Underground root-gall colonies are continuous.) Any elm
trees in the neighborhood?

The closest possible one would be in Golden Gate Park - a minimum of 2
blocks away.