i think its a combination of all the reasons given...
as i see it..
PCs were marketed as Business machines (International Business machines), business users invested huge amounts of money in computers.
Amiga was marketed as a Jack-of-all-trades, master-of-none, gaming, creativity, but less importantly to Commodore, business.
PC hardware was developed by many manufacters, more competition can create a competitive market.
Amiga hardware was all "in-house", and Commodore were slow to release hardware addons (HDs, High Density Drives, Soundcards etc.. etc..) business users want it all in the box, not addons to stick to the side of your computer!!!
PC software was boring... your Wordperfect... your Lotus 1.2.3... business users tend to buy their software to remain legal.
Amiga software was cool (games etc..)... a 16 year dude, doesn't care were he gets his software, there is no one looking over his shoulder to make sure he owns the license to play "Defender of the Crown".... Piracy (despite what you say can kill a market)
what really shocked me at the age of 16, was when i went to college...
... i was chatting about computers to my trainer, when he asked "What computer do you have?"
my answer "An Amiga"
he said "huh huh.. yeah the Amiga... thats a Toy computer"
"Bastard" i thought..
that pretty much sums up the feeling towards Amiga computers, if of course your not an Amiga scener, i tried to argue about better sound compared to a 386, cheaper etc.. etc.., but it all came back to hard-drives... the only thing i had in defence to that was the "rock solid, gui based OS" ... (-wasn't DOS a bastard-)...
...i wonder if i met that trainer today and asked him "Is there still an active scene about 386s"... the answer, i know, would be "no, but check out what my 3.06Ghz P4 can do compared to your A1200"...
like you all, i'm sure, i wish the Amiga was still in todays computer market... i don't think there is anything anyone could of done to prevent the Amiga going tits-up, i think you would have to go right back to the beginning and say"Nah you don't wanna do it like that... you wanna do it like this... killer business apps, aim at the business market... improve the hardware quickly etc.. etc.."
no single reason is the downfall of the Amiga...