I think judging the quality of coaching in the NBA is much harder to do than in any other sport. Is Jerry Sloan a good coach? He doesn't have any rings, but his winning percentage is comparable to D'Antonis (though over a longer period obviously, since D'Antoni hasn't been in the NBA as long). Outside of the Pistons of a few years ago, you almost always have to have one of the top 3 players of your generation to win a title.
Lets recap:
Heat-O'Neal, Wade and the zebras
Spurs-Duncan
Lakers-O'Neal and Bryant
Bulls- Jordan
Rockets- Hakeem
Pistons (80s)- Isiah
Celtics- Bird
Lakers-Magic
Has D'Antoni had any guys on quite that level? Probably not. Nash is quite good, but he's just not able to take over a game quite like those guys did. Especially with all the bad luck the Suns have had (JJs eye, Amares knee, the suspensions, etc).
I do think D'Antoni has coaching deficiencies, he's obviously stubborn to a fault and I'd wish he'd use his bench more in the regular season, but heck, don't you remember the Frank Johnson days? I'll take a 60%+ winning percentage every day of the week. I like how the Jazz ownership has kept Sloan around even though he's never reached the ultimate goal, they know winning consistently at a clip like he has is quite a difficult thing to do.
D'Antoni himself has said the journey and not the destination is the most important thing. If the Suns never win the title, I'm not going to stop being a fan. I have fun watching the Suns, and I thought thats what sports was about. Sure its frustrating when they lose, but thats what makes the wins even more sweet.
So I guess what I'm saying is, is D'Antoni as good as Popp? Maybe not, but D'Antoni never had Duncan either. If Amare can continue his current trajectory, I think the Suns will have titles at some point, but its on Amare's back, they'll go as far as he can take them.