Is tonearm bias a compromise, maybe a myth?

I can't say for certain, but I think that the stylus/cantilever pulls the tonearm along, so if those loads are balanced properly then it is going to be centered. I had a buddy who used the ET2 linear arm on his VPI table. It took him a few years to figure it out, but I recall him telling me that he had to level his table a certain way- slightly downwards, I think and it would track perfectly. He also said that he had to keep the air bearing tube perfectly clean. It would sound grainy if the tube had any dust on it.
Also, cartridges are different. I think some may have a different compliance in the horizontal direction than in the vertical direction, ie. the cantilever may be stiffer in the horizontal direction. So you may not see as much deflection horizontally on some cartridges. My Benz cartridge has what looks like an o-ring on the cantilever for a suspension. So the bias downward and slightly to one side is very apparent. But when riding on the vinyl it is centered. The cantilever goes through a hole in the housing which makes a good reference for seeing the cantilever position.
When I am setting up my tonearm and setting the HTA, I have the protractor on top of a record so that the arm is at the right height. Anti-skating is set to zero. I pull on the platter while viewing the protractor alignment. This applies friction drag to the stylus and sets the cantilever in the "as playing" condition. I can see the difference in the protractor alignment when turning the record slowly versus the record remaining stationary. (The record slides along under the protractor).

I just uploaded a couple of pictures of my phono cartridge for illustration on my
system page. One shot shows the cantilever in the free state and resting low
and to the right. btw- I said that wrong in my previous post about being down
and to the left. (I'm left handed. We get our rights and lefts mixed sometimes.)
The other shot shows the cantilever cued onto the record. I tried to get a good
angle. It shows the cantilever very nearly centered. The angle of the camera
shows it off a little, I think, but in reality it looks centered.
I usually end up at the high end of the VTF tolerance from the manufacturer and
my AS force set a little less than my VTF. I also keep the stylus clean with a
wetted stylus brush. That is key to maintaining good tracking.
I have a 45 rpm direct to disc record of the Appassionata that is very
challenging for any tonearm/cartridge. It is very dynamic. Once my old Koetsu
was worn I had to stop playing it. It could not track the record on the
crescendos. This cartridge tracks it perfectly from beginning to end and the
peaks do not come until the end. It gives me goosebumps.

Tony...as a fellow left-hander, I sympathise :)

You must have nerves of steel m8! ("DJ scratching" - in the "safe" direction - on your protractor :) :)
I'm usually a quivering wreck just making routine adjustments :)

It reminds me of NSGArch's repeated appeals that platters be secured during cart alignment to avoid any accidental cantilever reversals. Sadly, few of us - including myself - take this simple precaution (I will next time, I promise!) :)
On my turntable, I've noticed that after deceleration the platter comes to rest with a final small oscillation (backwards-forwards, about 1mm or so, for a split second). Perhaps wrongly, I attribute this to the rubber belt stretching alternately either side as it grapples with the heavy platter's inertia...
It doesn't encourage the idea of not being present during a power failure while a record is spinning! :)

Tony, When the stylus is tracking an actual LP, the friction that causes skating has everything to do with the grooves.

The main friction is between the vinyl and the stylus, but yes, groove modulation adds additional drag to the stylus and therefore creates additional AS force as Lewm points out. I found some data on the internet a few months ago. Research was done on various types of vinyl and shellac records decades ago. The coefficient of friction varied from 0.2 to 0.6. That coefficent number would have a direct impact on AS force along with groove modulation. I believe that most modern vinyl records have a fairly consistent coefficient of friction- probably in the 0.2-0.25 range, but that is only a conclusion based on that research data. Friction force is your normal force, eg. 2 grams, times the coefficient of friction. So if the coefficient is 0.2 then the friction force would be 2 grams times 0.2 or 0.4 grams and the AS force will be a component of that friction force. AS force is going to be friction force times the sine of the angle between the stylus and tonearm pivot, I believe. While tonearms may have AS force calibrated in grams for the sake of simplicity, the real AS force acting on the stylus is much smaller than that. Someone correct me if I am wrong with that conclusion. But that is why you really have to dial in AS force by listening rather than just setting it and forgetting it. The tonearm makers have to use average values while stylus angles, horizontal compliance and friction forces can vary which have a direct impact on AS forces.