I'm sorry, you are right, We rarely if ever wait in lines anymore and I've always done rider switch with a fast pass. So yes, that could slow it down a bit, and people cheating with fast passes to get more fast passes with rider switch makes it worse.
I'm trying to follow your comment here sorry bear with me here.
Why would people who need Rider Switch likely should have had fast passes anyway? Any person, who fits the qualifications, can use Rider Switch.
The description Disney uses on their website makes no mention of actual Fast Passes at all.
To put it another way here's the list of attractions where Rider Switch is available:
View attachment 265340
Now with MK you don't have tiers at this time but with Epcot you would only be able to get a FP for FEA, Soarin' OR TT not all 3 of them because all 3 of those are Tier 1 FPs (unless you get one as a 4th FP) despite Rider Switch being available for all 3 attractions.
Also for example we do know that 7DMT (as this was the original topic of the thread) can be a difficult FP to get so it is a possibility that even if a family knew they would need to use Rider Switch they may have been unable to get a FP for it.
Rider Switch availability isn't tied to FP availability as far as capacity per hour. From what I'm understanding, to Disney, Rider Switch and FP are not connected at least in the way I think you're thinking.