Actually, Armstrong had experience taking an elite roster and making it steadily worse and a chronic playoff underachiever. [Kind of sounds familiar.]
Yzerman had 4 years under Ken Holland, but also spent a decent amount of time learning from the side as a player. He also had Steve Stamkos on his roster, which is a pretty good building block to start with.
Sakic had about 3 years as an advisor before officially assuming the GM duties; even that final year when he was largely doing GM stuff, he had Greg Sherman helping and even now Sherman's around as needed. Who's going to fill that role if you bring Pronger in and make him the GM? Dave Taylor's about the only guy in the organization who has experience as a GM, but he's a DA hire; if you're cleaning out the front office does he get the boot too?
Well, I did once upon a time think Therrien would lead the Canadiens to a Cup. Once upon a time - you know, before he got canned by the Habs, then the Pens, then the Habs [again].
Keefe, ... someone elsewhere brought up a good point. Remember when Dallas Eakins was the hot name on the market because of his success with the Marlies? Then he went to Edmonton and ... well, he's not nearly the hot name he used to be. The thought is that Keefe looks great because it's the Marlies and Toronto can plow all kinds of resources into hockey ops; if you put him somewhere that can't freely lob those $, is he going to still look that good? I'm lobbing it out as food for thought, not to argue that he's not a guy to be considered.
That's narrowing the field to an incredibly small couple of guys and making the search unrealistic. Dean Lombardi would be the only choice available, but by all accounts he's tied to the Flyers for a couple years with a pretty iron-clad contract. If we're going to look for a guy with some past management experience who might fit well, I'd look at the following:
* Tom Fitzgerald, assistant GM in New Jersey. Not sure if he would want to leave New Jersey seeing as how his family has set roots there, but with Brodeur back in the Devils organization and Shero inching toward thin ice, but he's got a decent amount of experience [even if a large part of it is under Shero] and most reports think very highly of him. I think you give it a shot.
* Bill Zito, assistant GM in Columbus. He's been AGM there for a while now, and he's a former agent so he's familiar with both sides of the table. He's watched what Kekalainen has done building up the Blue Jackets, so he could bring some of that experience with him. Again, not sure he jumps Columbus for St. Louis but I think you give it a shot. [Note: Philadelphia is thought to be pursuing him.]
* Mike Vellucci, assistant GM in Carolina. Also the head coach at Charlotte [AHL]. He ran operations in Plymouth [OHL] and the Whalers were competitive year in and year out. [Also had good success with Detroit Compuware in the NAHL in the mid-90s, so maybe you also look at him as a head coaching candidate?]
* Bill Guerin, assistant GM in Pittsburgh. Highly regarded by many, considered a future GM somewhere, but seems to be the eventual heir to Jim Rutherford's position as GM.
Otherwise, if you're going off the board:
* Brian Burke. [Nah, just kidding - though I'd be tempted to bring him in for a year to be a special advisor to give a really critical look at the franchise and suggest improvements.]
* Pierre McGuire. [Nah, just kidding. Almost entirely.]