There are other issues as well though. Receivers often have a shorter life and become obsolete.
You mentioned 3 issues with AVR - 1) Power output, 2) Obsolescence, 3) Reliability.
1) Power Output. As I have shown, power output isn't an issue when compared to some standalone dedicated external Amps. So at least we can dispense with the power-debate because the amps in AVR (at least Denon/Marantz) are very capable, even down to 1-ohm with the 3000 series AVR.
2) Pre-pros are just as prone to obsolescence as AVRs. Pre-pros are nothing more than AVRs without the Amp section. Simply removing the amp section does not make them less prone to obsolescence.
3) Reliability is a lot trickier to prove.
I've seen plenty of complaints about pre-pros needing repair (Emotiva, Rotel, Marantz, Denon, Outlaw, and others).
Simply removing the unused Amp section of the AVR to make a Pre-pro doesn't guarantee that it will improve reliability in practice.
Even if Pre-pros are statistically more reliable than AVRs, it doesn't guarantee that any given Pre-pro will be more reliable.
Now if a Pre-pro (Marantz AV7703) were less expensive or about the same price as an AVR (Marantz SR7011/SR7012), then I think it makes more sense to buy the Pre-pro.
But if an AVR is a lot less expensive ($600 for the Denon X3400 vs $2200 for a Marantz AV7704), it is a lot more cost effective to buy the AVR to use either as an AVR or as a Pre-pro (using external amps). They will sound just as good as one another when used as a Pre-pro.
I have an 8YR old Denon AVR-3112 that has been flawless (been through many loud Karaoke parties). I have given my older Denon AVRs (also 3000 series) to other family members, and these Denon AVRs are still good today.
Of course, someone else could buy a Denon 3000 AVR series today and it could malfunction in 3 years when the warranty expires.
Someone else could buy a $10,000 Bryston Pre-pro today and it could malfunction in 5 years when the warranty expires.
One thing is for sure - it would hurt a lot less if a $600 AVR malfunctions in 3 YR vs a $10,000 Pre-pro malfunctions in 5 YR !!!