You miss the point. The parks don't necessarily do SR as a perk for guests (though it is a nice by-product). It's to fill empty seats on trains.
If the line to fill seats is an hour long, they'd have every right to shut down that line for an hour. It's served it's purpose until the line is empty.
Unfortunately, for many, it's become an unofficial Express pass in the minds of many, and that's a problem that brings entitlement into the equation.
I suspect it also works better for Potter because the trains are continuously coming with little stopage, and being limited to 4 seats, is less of an arguement for guests in stand-by lines trying to keep large parties together. However, on Mummy and JW, large parties will change their minds and rearrange their parties to stay together (eg, "Suzie wants to ride with her dad, not her sister!" "Wait, I don't want to be on the edge, I'll get wet, let's trade" "I can't see if I sit behind Uncle Joe, he's so tall", etc.), even after a grouper has assigned them rows, causing a lot of headaches for team members.