I don't think you can clearly say that Cowens was the #1 option / "#1 guy" on his team. The '74 and '76 teams were more of a three-headed monster of Cowens, Havlicek, and JoJo. Hondo was the '74 Finals MVP and the team's leading scorer; Jo Jo was the '76 Finals MVP, and Cowens was the '76 team's lead scorer by a very slim margin.
I guess I just don't think you can say "Hondo was a great second option". He finished in the top ten in scoring six times and in the top ten in assists seven times. He was the lead scorer on three championship teams, and was a key player on many more, winning eight rings in total. Also, when assessing "best Celtic", you have to look at longevity as well as dominance; Havlicek was at the top of his game for at least 12 seasons (12 All-Star appearances, 11 All-NBA), whereas Cowens had a shorter window.
EDIT: I agree that Cowens was part of the "heart and soul" of the team. However, Havlicek was, too.