I said it was possible for Disney to use FP+ to eliminate all standby lines by not allowing any guests in line that didn't have a reservation through the FP+ system. The key requirements were that people could choose their times and that they had a huge number of kiosks to accommodate all the guests having to make a FP+ reservation. FP+ has the ability to choose time and thanks to a very large number of guests having data phones, they now have a huge number of kiosks. If they understand the data correctly, they could allow the right number of FP+ reservations and window size to keep the line at 5 to 10 minutes all day long without the line ever getting to zero. That way no guest every has to stand "in line" for a ride for more than 10 minutes. Of course they will be standing around waiting between FP+ reservation times to get into that line. I am not suggesting this is a good idea, just that it could be done.
As far as ride capacity vs guests, only the slowest times of year have rides with no one in line at times during the day but then that is true whether you have standby or FP+ reservation system. All other times of year there is a line from open to close. Also Disney lowers ride capacity based upon crowds. BTMRR for example can run 4 trains at once but at the start of the day or during slow seasons they might only be running one, even when the wait time is over 20 minutes and could be reduced by adding another train. It would make the guests happy but it costs money for additional workers. So depending on which end of the shoulder season you are on, you could actually have to wait in line longer during a less crowded time because the crowd to ride capacity ratio is higher.