I'm not a police officer but I would suspect that in this day and age officers all over the country are faced with subduing suspects high on fentanyl and meth. I would not be surprised at all if this happened hundreds of times, maybe thousands of times, each day. And all those other suspects manage to make it to the police station alive and kicking.
No, kneeling on his neck was not the immediate cause of Floyd's death. But it was a contributing factor and one which the police were responsible for. Floyd may have died in their custody without being knelt on, we won't know. But he did die in their custody, their actions were a contributing factor in that, so they should be charged in his death.
Perhaps taking synthetic Heroin and resisting arrest contributed to his own death. Why are we bending over backwards to excuse the crimes of some to search for a crime by others?
Straining at gnats, we swallow camels.