No, your bike does NOT "freewheel" at 65 mph.
It's working as hard as it can, right up against whatever design feature that's holding it back
There's usually a relationship for engines and motors regarding how much torque they can make at a certain rpm.
So what's happening at 65 mph is that either it can't produce enough torque to overcome air drag any more, while theoretically being able to spin faster(if the load had been lower)
Or it can't spin any faster, while theoretically being able to deliver more torque( if the RPM had been lower)
Or, in a perfectly matched setup, it runs out of torque and RPM pretty much at the same time.
Downshifting CAN increase your speed on flat ground if you've got RPM left but no torque left.
Keep in mind that may equally well be the battery or controller that's the limit. Maybe your gearing ratio is OK, and you're already pulling all the amps the battery or controller can deliver, then that's it. Then you need another battery/controller to go faster.
My first guess is that it's torque/amps drawn from the battery that is your limiting factor.
Personally, I think you should stop being stupid.
If you want to go at motorcycle speeds, get a motorcycle.
An ebike, through and through designed to be safe and reliable at 65 mph would be expensive. And pretty much look like a motorcycle.
And judging by your posts, I don't think you have an expensive ebike.