In the modern game they're the most important, but in prior decades PGs or bigs have been more important. Most of the above had a wing and a big (think Bird/McHale or Havlicek/Russell).
I agree and disagree, the last 15 champs, all but GSW (3) and Toronto (1) had an impactful big on the team. This includes Duncan (2), Garnett, Pao Gasol (2), Dirk, Bosh (2), Love, Davis, Giannis. I don't think the impact of having a really good big and a really good wing has ever really changed. The GSW teams had Durant and/or Green, making up for the lack of traditional size. The Toronto team had Marc Gasol, Siakam, and Ibaka, not an individual top big but collectively, some good, skilled size.
Winning a championship without a top big is still more the exception than the rule. There was a stretch where the way they called fouls, it favored smaller players, but now that is changing. It is tilting back more to favor size. GSW are still winning with their main big being Green, who is not all that big and not what I would consider one of their best players, but every other top team has skilled size.