Just one experience and perspective ...
Having cycled up early this year - with a bit of walking too - going up is slow and hard, but no risk to other traffic. Not a lot of traffic anyway. On the way up, I passed 3 sets of road surfacing works, where the top surface had been skimmed off. No problem on the way up but it would have been suicidal for a bike on the way down, with no chance to keep safe control on loose scree on those inclines and bends.
The other aspect is brake types. I have rim brakes and descended the first few Kms to the first café (as they were all closed at the top). The danger with continuous braking on rim brakes is serious overheating which can cause inner tubes to burst. At the coffee stop I almost burnt my fingers checking the rims and then decided a Sawng Theow was the safer option down. My next bike will have disc brakes to avoid that problem!
But this decision by the park chief is a nanny state overreaction, restricting the untrammelled enjoyment of the Thai countryside. How many accidents have there been on the descent, and were the road works a factor? Let's have some facts Mr Park Chief. And support vehicles for all cyclists is completely unnecessary and adding to pollution. Who will bother to ride there again with those rules? Parks should be free and open to all.