The flaw in this logic is that the FP system is a "closed" system. There are a finite number of FPs, not an infinite number. So the 50 people who join the FP line in your example are 50 people who did NOT join the FP line during their stated time. So had you been in line at SM during the allotted time of those 50 people, the line would have been shorter because they weren't there. Sometimes you are the beneficiary of a shorter line and sometimes you might have a longer line. It all evens out. Simply stated, a longer line at time "A" equates to a shorter line at time "B". Since the system is closed and finite, you can never have a situation where the line is longer all the time. Also, while you are in line with the 50 people who do not "belong" there, it is equally possible that 50 additional people who "should" be there, are not. So the line might not be longer than it should at all. It might be the same length. Or it might be shorter. While the 50 people are in line at the "wrong time", if in fact 75 people who should be in the line are NOT, then you are actually coming out ahead.
The only time the system breaks down is if 100% of the people with FPs for a set time are in line, and additional people who are past their time join the line. But the probability of this is close to zero. In reality, the number of people in line who are past their time is offset by the number of people who are in the time window who do not get in line.