I've got 500 miles on my board and it has never been a problem, just something to be aware of. Adding mechanical brakes would involve one or two servos, two sets of brake disks and pads, and a few dozen more parts. That's a lot of hardware for a problem that few experience so it will likely never happen.

You already have a friction brake when riding a long board, it's your foot (with a shoe on it)

No shortage of air to move over the heatsink since you're only using the brakes if you're moving, so you shouldn't need a fan.

But there's definitely a fire risk if you're not careful. And you'd have people burning themselves when they pick up the board.