51% attack from Bitmain possible?

Bitmain has around 90% market share in bitcoin mining machines.

Bitmain controls Antpool wich produce roughly 26% of the world's hashrate.

Bitmain is known for having released a backdoor (Antbleed) and had other suspicious practices in the past.

What prevent them from using or disabling their hardware on competitor's rig to produce a 51% attack?

Not even a backdoor, but for example, a planned and precise obsolescence that would kill a shit load of machines on a defined date leaving Bitmain's free to take over or even kill the network if they are required to.

No FUD intent here. I'm a bitcoin maxi more than I ever been. But, this is the kind of questions I need to iron out from time to time.