While flying, the whole thing is suspended by the motors. It'd only be the weight of the centre components (mostly battery) that'd be flexing the arms.

(While crashing, all bets are off. But way back when I used to fly RC gliders they had a saying "If you build them to crash, they won't fly well. If you build them to fly well, they won't crash.")

Also, it's not like anyone expects thousands of hours of flight worthiness out of a consumer or even semi-pro drone. Sure the military might spec that for their Predator replacement, but most drone hobbyists I know end up with a bunch of perfectly serviceable old drones which they no longer fly because their gear became obsolete well before the mechanics failed irrepariably.

When resting, possibly. In flight, the weight of the battery would probably be the bigger concern, though I think you can get 2mm PCBs.

Thanks! this could be a great idea,the 2mm PCB could be an interesting option! We are devoted to 1.6mm PCB since the 5" version that weighs 220g and we haven't had any broken or issue on the PCB piece. The battery is well attached, fixed in 6 well distributed points on the PCB and aligned perfectly in the center of the 4 motors.

That's great then, if it works for you that's all that matters. I'm glad you guys are doing this.