The fact that my Denon AVP-A1HD pre-pro has already failed adds to your point.
The AVP-A1HD is a "separates" pre-pro with Class-A analog preamp section. Fully balanced from input to output and all that jazz. It has 2 giant transformers and a million other things. It weighs like a tank and is built like a tank until it breaks. It's so complex that Denon techs cannot fix. What good is that?
Yeah, bitter taste in my mouth.
So I bet the Denon X3400, X4400, and X6400 AVRs have a lot less internal parts than the AVP-A1HD.
Truth be told, one of the reasons I wanted to "experiment" with the Yamaha CX-A5100 was because I thought it might be more reliable than the Marantz AV8802 and AV8805 beyond 10 years. Only time will tell.