Pietrangelo would only not go up out of pettiness. He captained the only Cup winner and scored its GWG. He was the team's best defenseman for a decade and anchored a return from the dregs of missing playoffs to perennial playoff contention. Why even have jerseys go up if you're going to snub one of the best players the franchise has ever had and its only winner. The dude was on the ice for 18 of the Blues' 20 shots in Game 7. Imagine if they hadn't had him, they don't even sniff the Cup. The Cup is the most important thing to happen to the franchise in its history, by a lot. The team is still not a true contender because they lack a player like him. This is the definition of impact. This is a champion who's going into the Hall of Fame. I would throw up if they let someone else wear 27.
Petro needs to do something else major before he is a Hall of Famer. He's never been a finalist for any NHL award and he's never been a 1st team All Star. He doesn't have gaudy offensive stats to make a clear case as an offensive guy (his 499 points put him 199 shy of Pronger, who sits at 26th all time). I think Petro can get there if he wins a Norris or another Cup. But he won't get there unless he does something major in Vegas to build his resume.
Doug Wilson has a Norris win, was a finalist another year and racked up 827 points. Kevin Lowe has 6 Cups (and a King Clancy FWIW). Zubov has 2 Cups, a Norris finalist, and 771 career points. Pronger has a Cup, two other Finals appearances, a Norris and a Hart. Lidstrom has 4 Cups and 7 Norris wins. Housley has 1232 points and a Norris nomination. Blake has a Cup, a Norris win, 2 other Norris nominations and 777 career points. Niedermeyer has 4 Cups, a Conn Smythe, a Norris win, 2 other Norris nominations and 740 points. Chelios has 3 Cups, 3 Norris wins and 948 points. Mark Howe has 3 Norris nominations (all 2nd place finishes), 3 first Team NHL All Star nods, was the WHA Rookie of the Year, won 2 WHA championships, and finished with 742 NHL points to go with 504 WHA points. Leetch has a Cup, Conn Smythe, 2 Norris wins, a Calder and 1028 points. Stevens has 3 Cups, a Conn Smythe, 3 Norris nominations and 908 points. MacInnis has a Cup, a Conn Smythe, a Norris, 5 other Norris nominations and 1274 career points. Murphy has 4 Cups, 2 Norris nominations and 1217 points. Coffey has 4 Cups, 3 Norris wins, 3 other Norris nominations and 1531 points. Bourque has a Cup, 5 Norris wins, 10 other Norris nominations, and 1579 points. Langway has a Cup win, 2 Norris wins and another Norris nomination. Fetisov has 2 Cup wins as a player and another as an assistant coach, but is largely in due to his International resume (2 Olympic golds, 7 World Championship golds, a Canada Cup gold, and 3 World Junior golds).
That is every D man inducted into the Hall in the last 25 years. Every one of them has a better resume than Petro and that will not change if Petro reaches 700 points without winning another Cup or a major award. I think that #27 should eventually be retired here. Wearing the C for the franchise's first Cup is a hell of a culmination to a fantastic decade+ long career. He was the ice time leader for that run, was 3rd in points from our blueline, scored the Cup winning goal and was (at absolute worst) a top 5 contributor to the win (and he sure as hell wasn't 5th). Contribution on 1 playoff run isn't enough for retirement, but his excellent play over a decade puts him top 3 among franchise D in basically everything.
Among Blues D, Petro is 2nd in goals, 1st in assists, 2nd in points (just two shy of Al), 1st in even strength points and goals, 2nd in games played, 4th in +/-, 4th in PP goals, 5th in PP points, 2nd in shots, and 1st in minutes played (by 2000+ minutes). He's top 5 in all of these in the playoffs as well. At worst, his performance as a Blue was 3rd best at the position in the history of the franchise and he was here 100+ games longer than either of the two you could put ahead of him. And then he delivered the best performance of his career en route to being the first player to ever touch the Cup in a Blue Note. I don't think he's a Hall of Famer yet, but 27 should absolutely go into the rafters. It might take a new ownership/leadership group to get over the 2020 contract negotiations, but it needs to get there some day.
If the Blues traded everyone today, then I'd say Petro should be the no-doubt next number to get retired. No one else on the Cup roster has the resume he does with the team. Tarasenko isn't top 3 among forwards in any franchise stat and has zero chance of getting there unless he signs an extension and plays several more seasons here. Perron is 9th and 10th among forwards in most stats. ROR obviously hasn't been here long enough to climb the leaderboard. There are guys that have a chance to keep building their resumes to get into the rafters before Petro, but they are all currently behind him.