As a die-hard Lafleur/Lemieux/Habs fan, this would have been a dream scenario, but with the painful part of not having Lafleur on the team.
I'm going to go in a slightly different direction with how I think things would have gone in this scenario. As most have said, Mario would have developed a more well-rounded game in Montreal, making the team more of a threat overall in the mid 80's early in his career. Assuming the Habs still have Chelios, Roy, and Richer you would have a core that would make the Eastern Finals every year, if not the Stanley Cup. Under this scenario, I believe Gretzky and the Oilers step up their game, knowing that they have to be at their very best. Remember, they had just supplanted the Islanders dynasty, and I don't think they would want to surrender their place as top dog this early on. I believe the Oilers would have ran the table from 84 to 88 (being the second team to win 5 Cups in a row), until Gretzky was traded. Gretzky was always up to the task of facing Lemieux, and he seemed to fare better in head-to-head match-ups, even when he was on the inferior team. Cup finals featuring Gretzky and Lemieux would likely have gone down as some of the best series of all time. Even with Patrick Roy standing on his head, I believe that if those Oilers came to play, they would still have the advantage.
From 89 onwards, I think Montreal is good for 2-5 Cups depending on Lemieux's health, and the free agents and trades that go to Montreal. The Habs would likely not have developed the pure firepower of the early 90's Penguins who had Stevens, Jagr, Francis, Mullen, Tocchet, Recchi, Coffey, etc. But good teams and star players attract talent so it's fun to dream of whom Lemieux might have skated with in this scenario - Fedorov, Mogilny, Turgeon, Lafontaine, Hull, Neely, Bure, Forsberg, Lindros, Sakic, Sundin? I'm pretty sure one or two big names would have signed on with Montreal and turned a great team into an all-time dynasty.
Other dynamics that might have changed:
- During the 87 Canada Cup, maybe Lemieux plays even better, with a better all-around game and leadership qualities he would have acquired from all of the Habs Hall of Famers. He attributes improving his attitude towards the game to playing with Gretzky in 87, and he might have already been ahead of the curve had he started his career in Montreal.
- Lemieux had a lot of tantrums about all of the clutching and grabbing going on in the dead puck era (he felt the league should protect star players more like they do now). This, along with his bad back and cancer, made him retire early. The Habs, being an Original 6 team that was winning Stanley Cups, might have had more clout in being able to lobby the league/refs better. Or, if that was not successful, by having lunch or dinner regularly with guys like Rocket/Beliveau/Lafleur, they would have been able to convince him of his "responsibility to carry the torch" and he might not have retired early.
- Lemieux was not quite the point-getter that Gretzky was, and he might have even been less so in this scenario. However, in terms of goal scoring, he did seem to age better. If Montreal maintains a competitive team late in his career, Mario becomes a power play specialist, and they try to save him more for the playoffs every year. He retires very close to Gretzky's goal totals, and maybe even matches him.
- With Lemieux, Montreal is capable of fielding unquestionably the greatest line-up of all time. Right now, an all-time Montreal team is beatable because they don't have any of the Big 4, and their advantage at goalie is redundant (you can only play one of Dryden/Plante/Roy at a time).
This has been a fun exercise.