I don't think there's much Pierce could ever do at this point in his career to get any higher than 5th but, in all honesty, I don't think he'll ever get higher than 7th in my eyes.
Russell, Bird, Cousy, Havlicek are players he will never unseat so he has a chance at top 5.
In my eyes he may never surpass Cowens, McHale and Jones.
Now, I never saw Russell, Cousy or Jones play but being a Celtic fan, to me, meant getting to know the history of the team and I have read extensively about the team's history and so have a full appreciation for those three. I just don't see that their legacy's should be forgotten because the new guy that put up a ton of stats on bad teams got a bunch of younger fans to be Celtic fans.
Read up on your team history folks. Appreciate what those banners mean and who put them up there and what they had to persevere through to get them.
Cowens - two banners
McHale - one banner as a starter, two as a reserve
Sam Jones - third or fourth best player on a lot of championship teams
So I don't see how from a banner perspective Pierce's hypothetical two wouldn't be on par.
I am not trying top say that winning banners is the prerequisite for going higher on the list. I am saying those banners are there because of a lot of great Celtics and because the younger generation comes in and loves their players, they shouldn't forget or put their favorites ahead of others that might be more deserving simply because they are familiar with them.
Cowens was an MVP, a two time champion, a Rookie of the Year and one of the greatest undersized defensive centers ever to play the game. He guarded guys like Kareem Abdul-Jabbar and Artis Gilmore and Bob Lanier and Elmore Smith and Bill Walton and Moses Malone, men much bigger than him and shut them down. He is also one of only five players in the history of the NBA to lead his team in all five major statistical categories(points, rebounds, assists, steals and blocks) in one year. Sorry, I watched him play. He was a better player than Pierce will ever be.
McHale is also one of the greatest defensive PFs in the history of the game. Let's not forget that in the early 80's he was also the best Sixth Man in the NBA. During his peak, McHale was unrivaled on the court given his defensive presence and almost unstoppable low post presence. He was probably after Bird and Magic the best player in the league for a couple of years. Pierce never reached that height of greatness and never will.
Sam Jones was a pioneer at the shooting guard position. He was one of the first "big" SGs that would also crash the boards and finish at the basket. His game and style of play transform the position of shooting guard. Add to that that he was one of the most deadly shooters in the NBA at his time and also one of the most clutch players in NBA history and one can see that he was a lot more than just the "3rd or 4th player best player on a lot of championship teams".