Let me repeat: IT COSTS NOTHING!
It costs NOTHING to have a way to bypass the preamp and go directly into the amplifier. Receivers used to connect the two via jumper bars on the back the user could remove. WHY WAS THIS PRACTICE EVER DISCONTINUED?!
It's ridiculous!
The answer is competition for "real estate" on the back of the receiver.
Honestly though not many would use an old receiver just for the amps. They are not good power amps anyway, low end at best and that could be over stating it.
If you want power amps, then go separates. That is your answer. I agree there is obsolescence. My power amps are on on their fourth AVP going back to pre HDMI.
That is you answer, not plugging your gear into half a boat anchor.
Anyhow if you really wanted to do it, you could easily mod an old receiver, and connect the amps to some of the old RCA inputs which would not be used. More often than not though, old receivers go into protection and give you the red light of death, so access to the power amps like you suggest, would not help. Your solution is separates with big boy power amps, and not those puny receiver amps, on "postage stamp" circuit boards.