I think the best proof of Marv's greatness of a head coach was his ability to get the team into the Super Bowl year after year. I'd think after at least the second one, most coaches would fail to get the players to regroup. Yet he brought them back four times.
This is in line with the two comebacks. Everyone remembers the Oilers comeback, but remember the 1997 Colts comeback as well. With Todd Collins as QB, Marv was able to get the team to regroup and make a very impressive comeback by a substandard team.
I think there were some mistakes in the Giants game that we can point to, but in any close game there always will be. The Cowboys teams were stacked and were successful the rest of the decade. I feel like the last Super Bowl was still very winnable, but the Cowboys clearly had the better talent.
If the team had had a DC like Wade during the Super Bowl years, I think we would have won it.