Fact is one is going to be a better coach than the other, right? Some coaches might just be "good" rather than "excellent", and that difference might not always be down to simply being victims of circumstance.
If you give 18 coaches the exact same list with the same injuries and form slumps, across 3 years someone is going to come last, someone is going to excel, most will be middle ground. IMO only half a dozen of them will have a shot at the flag (I reckon it's a fair call that a premiership will typically only won by one of the top 6 coaches in the league). The question is, would McCartney have his team in the top 6 of that comp? Take out the development that every coach has hypothetically had a chance to imprint on the team for 2-3-5 years, just basing it purely on results with the exact same tools at their disposal, where does he sit.
People make the argument "well we're young, so his crappy results shouldn't count against him", but I see someone like Grant Thomas - he takes over a team that is an absolute rabble, a team that has two wins for the entire year. Quite a young list that's been battered, and within 3 years, has them sitting 10-0, on the way to an interstate prelim final where they lost to the eventual premiers by a kick. Yeah, a couple of old heads in the team (much like we've had), but look at the St Kilda bests in most of their games in 2004 and the majority are guys 22 and under.
I look at Clarkson, Lyon, Hinkley, and ask what if they had taken over in 2012? Would it be some miracle turnaround compared to now? No. But would they have a better win percentage than 30%? Would three of the unquestionably best coaches in the league have us in a better position? My gut instinct says yes. It might only be an extra 7-8 wins across that 3 year time frame, but that bumps up the winning percentage to 40%-45%. You might say, "ah, 30% or 40%, no biggie", but that one, two, three games a year that a coach's decisions/tactics/strategy/list management is what might dictate the difference between making and not making the 8 in two years, between winning and losing prelims in 3 years, between winning and losing a Grand Final in 5 years.
I can't definitively compare where we would be if we went for Sheedy instead of Hart in 1980 "because it didn't" either, but am I allowed to hazard a guess that things might have turned out differently?