There’s nothing magical about a ‘GM’ in terms of selecting the groceries. They are just as capable of crippling mistakes as any HC is. But you either have to a) have a HC who you are confident is going to be there for a decade (an Andy Reid, Bill Belichick, or Mike Tomlin type) or b) you have to have a strong GM. The reason I say that is, with a HC given all decision-making power, if he is not a lock to be a long-term guy, he can be making decisions that are great for HIM and his success in the short term, but terrible for the prospects of long term franchise success. In most cases, you need that GM to ensure that doesn’t happen.
We can debate whether there are candidates like Sean Payton who can be relied on to own both hats and serve both short term and long term interests well. In the vast majority of cases, I think a GM is critical.