"the computer knows 90% of the time exactly what is wrong with your 21st century car, but the techs may be losing the basics in the bargain, resorting to part-swap diagnosis in those 10% cases."

There is no "may" about it. Not all have lost the ability but if you frequent any of the boards regarding some of these newer cars you will find out there are known common failures where the failure is NOT specific to the part indicated by the OBDII (or manufacturer specific system that goes beyond ODBII).

No where is this more obvious than when you cruise the BMW boards and look at posts related to Vanos system failures. I was considering a later model wagon not that long ago and the BMW 3- and 5-series wagons were high on my list. I had a Wife-Approved spending limit of the $7000 so some cars that would normally be out of my reach suddenly were an option. I looked at one poor schmuck's 2000'ish 5-series wagon that he was selling for way too cheap. Riffling through the BMW DEALER repair records he had more than $4000's in Vanos related repairs and still was throwing codes. A quick search of some of the BMW-version of the Brickboard brought up three no-to-low-cost tests that could be done to determine the root of the code.

In the end I bought my 940 wagon but I did send the the perspective seller the links to the tests as well as the name of a (as reported on the same site) reputable independant BMW specialist.