Stephen Harper is a relatively young man, especially for someone who could be Prime Minister in less than a month.
He's 46. Meaning, there's no way that he could retire because of old-age (Trudeau and Chretien both basically went out this way).
Considering that it's easily possible to serve as Prime Minister into one's 60s and even 70s (Chretien was just short of his 70th birthday when he retired), it's interesting to speculate on how Harper will leave his post as Prime Minister (if he even attains that post.)
I think that the actual title of Prime Minister will raise Harper's stature considerably. Once people see him in the Prime Minister's chair, once people see him putting his agenda into action, I can't think of any federal Liberal (out of all the potential leadership candidates) beating him in an election.
Also, Harper is far different when compared to Prime Ministers from the past, both Liberal and Conservative. Trudeau, Mulroney and Chretien, all 3 of these men were fairly occupied with attaining and maintaining their power. Harper, at least in my eyes, doesn't come across that way. I think that this quality will make Harper hard to defeat. I think he'll be shrewd in maintaining his post as Prime Minister, but he won't be as transparently power-obsessed as most past Prime Ministers and Prime Ministerial contenders.
Anyways, moving onto potential downfalls for Harper, let me go over the possibilities and why I find them unlikely:
1) Being defeated in the House, and then subsequently losing the general election: At least in the short to medium range future, I can't see this happening. Paul Martin is fairly old, he's in his late 60s. A Liberal loss will almost surely result in a new leader, but who? Every potential candidate seems highly mediocre, both in their personal charisma, but also in their electoral prospects. None of the major contenders are francophones (meaning the Liberals will likely be unable to recapture their Quebec base), and none of the potential candidates really has a national profile. Again, like I said, I think being the incumbent will help Harper a lot in future elections. He won't sound whiney, he'll actually BE Prime Ministerial, instead of seeming like it, and he'll also likely have a somewhat impressive record to point to.
2) Being challenged from within: If Harper wins the election, his leadership within the party will be unquestioned. He'll be a hero to Conservatives across the country. Some will undoubtedly wish for a leadership race, but they won't get one, as long as Harper sits in the Prime Minister's chair, even in a minority parliament. Stronach, sitting in a Conservative cabinet, would have undoubtedly been a major annoyance, as her only aim would have been for Harper's job. Her defection this past spring may prove to be a major blessing in disguise.
3) What else is there? The first option presumes that nothing "major" will happen. I'm assuming that the Conservatives will put forth and largely pass laws if they're the government, and that they will eventually be defeated in the House. National unity issues are still pretty out there, but the Liberals have lost all credibility on that file. The economy, which was largely Mulroney's downfall, will likely continue to be in good shape, so that issue shouldn't cause any trouble.
So assuming that Harper DOES become Prime Minister, for how long will he hold that post? There's no major obstacles lying in his way (except for national unity, but like I just said, the Liberals no longer have the upper hand on that file), I can't imagine him being defeated in a general election by any one of the current batch of Liberal leadership contenders, and his leadership from within the party will almost certainly be unquestioned.
There's so many ifs, especially when predicting what will happen in the years to come, but if Harper DOES win the election on the 23rd, I think that's he's likely to be Prime Minister for a Trudeau or Chretien-like period of time. Once (if) he's in the PM's chair, I think he's smart enough to not make any stupid mistakes, and as I've tried to explain, a lot of other things are lining up in his favour as well.
Let's just hope that if I'm right about Harper being in power for an extended period of time, he uses his time in office productively, fostering the creation of a long-term and genuinely conservative Conservative majority in Canada, something I'm sure Harper considers his ultimate goal and potential legacy.