> curve in datasheet indeed doesnt match minimum beta specs.
The printed spec is guaranteed... if it does not meet that spec (with all footnotes), it is "bad" and you can ask to replace it. If you just have one bad part, it is not worth the effort; if you buy a million parts you save your duds and bring them to the negotiation for your next million parts, maybe get some allowance.
The curves are "Typical". They show trends. If the average performance is more important than the worst-case performance, they may be useful. Interesting, but any critical design must be based on the Guaranteed specs. Maybe they get Hfe=150 most of the time, but 55 on a bad day.... if the Spec says "50" then they can sell them under that spec.
> 3904s and 3906s have seemingly gotten better over the years
I think they have far fewer "55" days. Or they make so many, so cheap, they can afford to throw-out a low-Hfe batch. Or they have side-contracts who will buy low-Hfe parts if the price is low.
> the beta falloff with lower collector current is {was} pretty foul.
Reasonable for that to improve with improved processing.
> neither particularly high beta nor low rbb'.*
You been in the racket long enough to recall what we got just a few years before. Transistors were more like thermometers, and Noise Figure below 20dB-10dB was worthy of note.
The 2N39__ parts were a lot nicer to work with than the older cheap parts. Like the 2N2219/2222, only cheaper.
"Low Noise" must be qualified. For audio, for 1K-10K, these parts ain't bad. Often a bunch better than a tube.
> why not matched 4401/4403?
Because only microphone inputs need anything that beefy. OK, other things do too, but many-many more chores can use the smaller parts, get more product per wafer.
OTOH, ON Semi also makes big BJTs with onboard bias diodes, pretty much an audio-only thing, so they are not deaf to audio product needs. Maybe a fatter pair is on the schedule. Or maybe they feel THAT Corp and the LM394 sew-up the market. Product Marketing is not always rational. Or "taking up resources that could be used more profitably."