It is a weird scenario. Generally when there is a low level of excitement it also involves a low level of interest and concern by voters.
But it is not like that now. There is a HIGH level of concern within both parties but also a HIGH level of resignation about the inevitability of the lead candidates.
But any sign of real blood in the water or real vulnerability would turn the whole thing into a vicious sharkfest.
I guess you were talking about the general election though. But I will similarly say that it could be a sleeper until it isn't. If neither candidate can get 270 because a third party has bled off some electoral votes (and the vote goes to the House) that WILL NOT be a sleeper.
Also Joe could go tits up at any time before or after the nomination which would also NOT BE sleeper. Joe is not going to make it through two full terms - even one is questionable. A vote for Joe is a vote for a constitutional crisis. But for those who think like dems rather than Americans, that's fine. They just want the D after the name of the President.
I am speaking about the general but the enthusiasm is low in the primary too. On the very real but outside chance Biden should not be able to perform his duties there is a constitutional remedy in the VP. As I have always said when it comes to these things the vice president’s health is actually more important than the president’s. While you worry about Biden’s health the better worry is his frailty. Trump on the other hand his cardiovascular heath is an actual issue considering he has heart disease for which he needs medication. But if we are talking about thinking like an actual American this is exactly why we moved to have the President and VP on the same ballot - for continuity of government.
Now should Biden or Trump have a health issue on the campaign, yes, absolutely that will shake up the election. You often talk about unknown unknowns but that works both ways. Don’t think your leading candidate is in the green zone.