Again, the question is which sports is "hardest" and the question is subjective in nature. There isn't an objective statistic that can show one over another, including the one you think you provided. Less varied titles among teams does not equal harder to win.
Less games to win to claim a title would in effect make the probability of varied winners more likely, not less. However, in the NBA changing the number of games in the first round has done nothing to help more varied champions. The reason the NBA increased the first round was not only for a few extra bucks, but to give top seeds a better chance of advancing to the next round.
Simply, having to win fewer games in order to win a championship is easier. I am surprised you have such a tough time accepting this, especially since people are not robots. Fewer games = less strenuous = easier. We're talking about playing sports, not channel surfing. Playing sports takes a toll on bodies and minds. Therefore, the more one person has to do it, the harder it is.
Implementing a cap would only increase the chances of parity, not decrease it. Teams that can spend as much as they want on whomever they want would be in the best positions to destroy parity. When everyone plays on the same spending field thanks to a cap (which by the way, I totally support in all sports), parity should increase (it does in the NFL and the NHL)
A cap, in residual effect, ought to increase parity. However, when the range between the lowest spender, using 2009 numbers, (LAC 32mil) and the largest spender (CLE 116mil) is so vast, the residual effect of parity does not come to fruition. Also, owners have to actually have the cash in order to spend. Larger markets, generally, have the cash. Also, out of all of the majors that have a cap, it's acceptable to exceed the cap in the NBA providing it's for current players (Byrd).
The NFL has an unbalanced schedule based upon the prior year's record (none of the other sports have schedules created as such), which I think, fosters parity within the NFL more than its salary cap.
How does the physically grueling aspect make it easier or harder to win a championship in a given sport?

Again, it's human beings who play sports, not robots. Is this another example of how video games have skewed the views of life/sports?
Was this an extreme example, certainly. However, there is no objective evidence to a flat earth and though asked, no one has supplied any objective evidence to support any of the other three sports.
I recognize you haven't and I will also state I have yet to see any objective evidence that supports any of the four sports. There is no such thing.
Basketball is less of a team game and not as physically grueling are subjective points you are making.
While yes these points are subjective, they're far from wrong.
One should be closed minded on the truth. I am pretty closed on this subject matter unless some sort of objective evidence can be brought forth. Unless I missed it, none has so far... I look forward to something objective that supports the NHL, MLB, or NFL answer.
And just as a reminder. I am biased toward MLB.
I appreciate your candidness that you are biased towards MLB. I haven't voted for an answer because ultimately, I think comparing all of them are apples to oranges, and there is not one single true answer. Each of these sports requires talent and breaks (whether in the form of bounces or in officiating) in order to win a championship and there's far too many intricacies in each to do come up with a true objective answer.
The largest problem, as I see it, with your position is that truth is objective and absolute.