Success is hard to define.
If you define success as winning the league and/or champions league, sure thats definitive success but its also a very very long way away from where you guys are at. Sure it could happen, but it'd be by luck and it'd be short-lived.
I think success over the next 5 years should sadly be defined as consistently being in the champions league knockout stages and being in the top 4. The current football landscape is so different to what it was 10-15 years ago, and during the time SAF has been away, United have been on a downward spiral. Now in the league, we face two teams with debatably the best attacks in world football, playing the most aggressive attacking football the league has ever seen.
Its a huge task to be able to compete with those two clubs [unless they self implode due to poor transfer decisions or losing their respective managers]. Thats not to mention Arsenal, Chelsea, Spurs whom are all vying for the top 4 places too; backed with great managers, some world class players and decent resources.
Ole also has to cull the dressing room of the mentality, primadonnas and poisons who were prepared to down tools under Jose. No top-class player at a top-class football club should EVER down-tools. We do see it, and those clubs and players normally go through periods of relative success and now relative medicority (see: Chelsea (Hazard, Fabregas et al)). Having top class personalities in a dressing room to curb poisonous influences is VERY important, and once they leave, results normally turn sour (see: Madrid pre and post Ronaldo).
Ole can be successful but expectations need to be in check. He has to be given one to two seasons to get rid of the deadwood. Otherwise you're just putting pressure on a manager to once again provide immediate results, and leave in his wake a poor squad (Mourinho, Van Gaal, Moyes).
If I were a United fan; personally I'd be more interested in just having my club function the United way rather than immediately thinking about trophies. Sure, short term thinking can get you the odd premier league title (see: Chelsea) but you're left with a hollow, unlikable, plastic, almost disgusting club which will not be able to sustain its success because you're left with a set of players who decide when they'll turn up. People seem to think these are new generations of players, but that only tells one side of the story. There has ALWAYS in the history of football, been players with ridiculous talent whom had motivation issues and therefore didn't have the personality to play at a huge massive club like Manchester United (Riquelme, Robinho, Di Canio etc.). I think if Ole has you playing exciting proper football and firmly in the top 4 spaces for the next 2 years, its a great foundation to then move forwards.
If the fans start questioning him every step of the way, it becomes too difficult to work under. Make no mistake, some of the players you have are somehow silently running the dressing room and it has to stop if you want to reach the top again. They got rid of Jose but getting rid of Ole will be much harder as long as the fans back him.