I think that is a reasonable assumption. By the way, I don't think you will find too many mid range (based on price level) power amps with gains lower than 28 dB. Below are some examples:
ATI - 28 dB
Anthem - 29 dB
Outlaw - 28 - 29 dB
NAD C series - 29 dB
Emotiva Gen 3 - 29 dB
Monolith - 28 dB (No surprise, same as ATI's)
Those with lower gains such as Benmark's ABH2 are rated 100 W or less so they don't need as much. I don't anyone would not pair the ABH2 with a real preamp anyway.
Yeah, I agree that most people who buy a Yamaha A880-series don't intend to buy external amps.
I believe that most people who buy AVRs don't need external amps because they don't usually listen loud enough, sit far enough, or own very expensive speakers with impedance that dip too far south.
The A860 can output 100W x 2Ch into 8 ohms 20Hz-20kHz at 0.1% THD (AH lab).
We know the All Channels Driven (ACD) test isn't indicative of real would applications.
For most people who don't listen to bleeding volumes in typical rooms of no larger than 18' x 20', I am confident that most people don't need more power than what the A880 is capable of.
But if they do need more power and want to buy external amps, please get the A1080, 2080, 3080 to be safe.
Or better yet, please get the CX-A5200 with a 4Vrms (unbalanced) and 8Vrms (balanced) if you want to get external amps.