What is missing is how much current licenses are used. With that data
we could guesstimate expirations. That would take a study that the FCC
doesn't have the resources to fund and one the ARRL may not want to
know the answer to.

There may be a point in the future where the new loss of members begins
to increase but there is no way to forecast that because licenses are
good for 10 years and there is no way to project future expirations
because we don't have any idea what current license usage is.

Measuring how crowded the bands are? But that would require
measurements from years ago to mean much. Assuming that most
hams using their license spend about 2 to 3% time transmitting and
the rest listening (tuning around the bands looking for interesting
DX or rag chews) one could get a rough idea how many active
hams exist. Do one measurement on a contest weekend, and another
on a non contest weekend. From a QTH in the midwest. One would
have to figure how many hams are in propagation range at the
time of measurement.