Think of it this way.
Today with FP, you need to arrive at the front gate before opening, walk quickly to your #1 FP attraction, leaving your family behind so you can get a decent FP return time. Then, watch your printed FP window so you can sprint over to the next FP attraction before rejoining you family to experience your first FP experience. Repeat cycle.
One of the things that should be mentioned here (and I'm not, in any way disagreeing with you), is that you really have to consider each park one by one in this example. I think your example works best for Hollywood Studios, when literally almost every person gets in the gate and walks straight to Toy Story Mania. Whether to get a fastpass or to get in line before it's ridiculous. With FP+ People might have booked an early Fastpass for Star Tours instead, and that completely changes their morning habits.
Whereas at a park like Magic Kingdom, you might get there early see that Space Mountain is still at a 10 minute line, ride it now, and rebook your FP+ for Jungle Cruise after lunch. Or at Animal Kingdom you might do things in a different order. Everyone goes in and heads straight for the Safari, but if they have a FP+ For The 9:30 Lion King, they probably won't rush and head to Lion King first and do the safari later.
Basically the point that I'm making here is that regardless of Capacity, it should help spread out the crowds a little better, yes they will still be there, but likely at different times then we have been experiencing over the past few years.
In terms of the actual FP+ line, there is no information to suggest one way or the other. It all depends on whether WDW issues more or less FP+ per attraction.
....
At this point, we don't know if Disney intends to increase or decrease the percentage of an attraction's capacity allocated to FP+. All things considered, I suspect they'll stick with their current allocations since they have a lot of data collected and, I assume, have already optimized it. Again though, we'll have to wait and see.
I realize that what I'm about to say is not sourced so I expect flaming.
Disclaimer aside, the FP numbers now are not changing. Throughout last summer Fastpass Numbers were tinkered with at all four parks, and even the operation of some attractions were tinkered with to make Fastpass lines move faster in an effort to stretch the current Fastpass System to it's limits. The current plan for the changeover is to draw FP+ Reservations from the current pool of allotted Fastpasses. Obviously this will grow and change, but the numbers have already been tinkered with.
So in the opinion of all you knowledgeable people, will FP+ lines be longer, shorter, or the same as the existing FP lines???
So if you read my above, you should infer that they will be about when they have been for the last six months or so.
Also just as an FYI for everyone the current plan is that FP+ reservations will be available 60 days out. So we can all stop making the 180 days jokes...