It's time for some more math! Let's explore the possibility of running standard FP and FP+ at the same time. For now, let's just look at one extremely popular attraction; we'll use Space Mountain. These calculations are based on those found in my previous post.
I don't know the specific numbers, but I've heard estimates of Space Mountain's hourly capacity being somewhere near 2,000 riders. Now, we'll again assume Disney is using twelve slots, so well divide 2,000 by 12 to get 165 (rounded down from 166.666 because you can't go above capacity), so that means that in every slot you can only have a total of 165 potential riders. Now, assuming every FP+ user who has a FP+ within that particular slot returns around the same time, you have 100 people now able to ride the attraction in the FP+ queue and only 65 people able to ride in the standby queue. To decrease FP+ wait times, many more FP+s holders will be let on to the attraction then standbyers.
To further put that into perspective, each hour 1,200 FP+ users will be able to ride while only 800 standby guests will be able to ride. Imagine WDW also distributes 300 standard FPs each hour (so 25 FPs per slot). Now, you have only 25% of hourly riders coming from the standby queue.
So, while Disney could use FP and FP+ together, standby lines would skyrocket. That being said, we don't know if there will be a decrease (or increase?) in guests using the standby queue after FP+ is implemented.