Bolt on cogs

Yes appreciated, but you never drive your wheel with your disc brakes... only stop.

I know bolts are extremely strong etc esp. 8.8 or 10.9 tensile. Just seems a bit backward and more effort than buying a 120/130/135mm fixed hub.

Compatability becomes an issue with having to use a 3/8, 9.5mm or 10mm rear axle in a front hub, sounds a bit heath robinson.

Yeah and stopping will put just as much force on the hubs as you and your legs can. A lot of people on here seem to have funny ideas about the amount of force that is generated riding fixed compared to any other form of riding.