Not sure if you were running a stock power cord or succumbed to temptation?
Thou shall resist temptation and not stick the serpents of Eden into thy plastic receptacle...
Thou shall not covet thy neighbors amp in temptation of its more exotic parts...
In all seriousness, there was a thread recently about shoving heavy cords into those receptacles and even using various materials to make a tighter fit after they stretch the plastic? Why be surprised then when a plastic part cracks/fails?
That is definitely not the fault of McIntosh if an aftermarket cord that put too much strain on the receptacle was used.
We also discussed that McIntosh does not use fancy "here today, gone tomorrow" parts. So how convenient is it that you can order that part from a parts source instead of it being some limited run of boutique parts never to be found in another few years?
Seems like an easy enough fix and perhaps get a spare or two just in case. I know those serpents will try to snake their way back in there....