The simple fact is that MON and Lerner are equally to blame for the carnage that has been the last 3 seasons.
MON identified the players he wanted and his turning the team from 16th to 11th to 6th over 2 seasons showed to Randy that here was a man who could make progress. So Randy backed him. MON delivered 6th place again and some decent cup runs but it looked like with another push was needed to break into the CL. So Randy backed him again. MON got 6th again.
The problems with MON however were now there for all to see. Players knackered by March due to an inability to rotate. Appalling man management of some, and an appalling overestimation of the worth of others (in terms of wages, fees and ability). Plus the scandal in the Uefa Cup.
Given all that Randy pulled the plug and said no to MON. Hence the hissy fit that saw him walk out.
Randy backed a manager he shouldn't have for longer than he should have. We didn't care because we were "successful" but in hindsight the mistakes are pretty clear for all to see - and we are still paying for it now. Problem is, Randy has gone too far the other way, he now won't invest for fear of getting that badly burnt again.