I think people want to blame someone and the coach is the obvious easy choice.
Bottom line is that there is no chance in hell that Igor is a bad coach. From Serbia, to Georgia, and Slovenia, he is considered an outstanding coach in Europe. In the states, he has been highly regarded everywhere he's coached- Clippers, Detroit, Phoenix, Utah, etc. I am going to assume that Alvin Gentry, Larry Brown, and Quinn Snyder are better judges of coaching abilities than a few malcontent fans on a Phoenix Suns message board.
I think that one of the problems is that Igor is very methodical and not reactionary. Casual fans (myself included) sometimes like reactionary. Igor started the season trying to see what he has with the veterans. Theoretically, Ariza, Anderson, and Canaan should have been better than a bunch of rookies. Without looking it up, there is 30 seasons of NBA experience between them. He stuck with the vets for a few games and then started making adjustments, one by one.
Igor will be fine. My guess is that an experienced, calm, methodical coach is exactly what a young team like Phoenix needs. Fans want someone who will get angry, someone who will yell, and scream, but anyone who's ever been in a leadership position knows that anger, yelling, and screaming rarely works. That's a tactic for some very rare instances in which it is helpful only because it shows the contrast with the "usual" approach. "Calm" is not "weak". You will notice that when Bridges got frustrated, Igor didn't let him just go to the bench. Igor was going to coach him (like he coaches everyone when they go to the bench) no matter what.
Igor knows what he is doing. He just has a team full of new players and rookies that lacks NBA talent. It will take a while for everything to get worked out, and I suspect fans will be calling for Igor to be fired after every couple of losses. That's a distraction. Outside of a few random games since the awful start, the Suns haven't looked like a joke, with the exception of those first two games without Booker and Warren.
There's a happy medium between yelling and screaming, on one hand, and complete lack of passion and listlessness, and inability to lead and communicate as a head coach, on the other hand, just as there is a difference between being methodical and just being plain slow and passive.
And for all of the progress Ayton has made defensively, he is still regressing offensively and on the boards, and the other incredibly coachable young players in Jackson and Bridges have regressed horribly (in Jackson's case) and zoned Kokoskov out after having regressed (in Bridges' case).
I have no doubt that Igor Kokoskov is an excellent assistant coach, as long as someone else takes the reins and is there to lead and communicate. For all of the years of experience you cite, outside of EuroBasket (where he could coach in his native language, which confirms to me that a lot of this likely is the language barrier), Igor Kokoskov has never been a head coach, and to me it remains quite clear why that is the case.
Pair Igor Kokoskov with a Larry Brown, Alvin Gentry or Quin Snyder, and sure you can have success. But you need the Larry Brown, Alvin Gentry or Quin Snyder at the helm to drive the proverbial vehicle that is the team. Put the proverbial navigator in Igor Kokoskov at the proverbial wheel, and that navigator might know where he wants to go, but he does not have the capability of driving the proverbial vehicle there.
Just my two cents.