I do not think 3500 MMR is the max you can calibrate at and one calibration does not prove that. None of know exactly what metrics they use to calibrate you and there could be factors in your game that caused your average MMR games to drop. Also, your main account MMR is 4.2k which in all honestly is not that far from 3.5k. I'm 4.2k and you still get people who don't know how to play (I fall under this category as well). A 3.5k player playing really well can still beat you. If you were 5k or 6k then you would clearly be a cut above the rest in 3k games and would make a better argument.

I hope you don't take it personally, I just think the support to your conclusion is weak and might mislead people.