And I am in NO WAY denigrating one of the greatest coaches.
But he didn't coach under the cap and DeBartolo spent about 30% more than any other owner did during that great run. He spent like the Yankees did and didn't care if the 49ers lost money. In fact I think I once read that the 49ers made the least money in the 80's because of how they spent to get talent in free agency and to keep players they drafted.
Belichik has had to navigate a different era with harsher challenges than Walsh or Lombardi and WAY more money available to players to lure them away.