If you have 1000 people in standby, and the ride capacity is 1000 pph, then getting in the end of the line, it will be an hour before you ride.
If you then give FP to 800 of those people, those people are no longer in the standby line. The standby line now contains 200 people, but it still takes an hour. Much shorter line as in number of people, but same wait time.
Logistics rather than wait time. There's a lot of complications with the FP system. Also, probably to avoid the situation of the anytime FP if the ride goes down for an extended period during the early days.
I can't speak to DL because I've not seen the data, but the data for WDW that Len's team compiled shows that on aggregate wait times did not increase on aggregate when FP+ was introduced. Some rides got a little longer, many rides (especially E ticket) got shorter especially at the peak of the day. But as a whole lines stayed the same and were more distributed throughout the day.
See my example above. The standby line does actually move slower because of FP but wait times are about the same because fewer people are in standby. The data at least at WDW bears this out.
But let's not turn this into another FP debate thread. If anyone wants to debate it further feel free to PM me.
Now back on topic: can't wait for RotR!
The fallacy in your system is that all of those people would be riding that attraction were they not able to obtain a Fastpass. So while yes, in theory, you're correct for peak attractions during peak times, the tier system incentives people to to ride things they might not want to otherwise.