While we don't know how many people will actually have Fastpass+, people tend to make that assumption based on the current Fastpass model. But what people don't realize is that, since at least 75% of the ride's capacity is devoted to the Fastpass line, when you get in a standby line, more people than would even be possible are riding before you than if there were only one line for everyone.
Lets say you queue up for something in the standby line and there are 1,000 people in front of you. If that was the only line, this is a set number of people ahead of you in line. It can't change. With an additional Fastpass line that takes (at a minimum) 75% of the ride's capacity, an indefinite number of people are riding before you in addition to the people who are physically in front of you in the standby line. Basically, the number of people that enter and ride before you due to having priority through using the Fastpass line is higher than would ever even be possible with only one line.