Anything will bend with enough force, look after your device and it won’t bend. Goes for iPhones and iPads. I’ve had all the “prone to bending” Apple products and not one of them has ever bent out of shape. Use a case, don’t sit on them or force them into bags etc and you’ll be fine.
The iPad is bending due to manufacturing stresses, not because the side edges are straight. A straight sided edge is more impervious to bending than a curved edge, so if anything, the 12 series (if they have square edges) will likely be a little more rigid than the current models (if everything else stays the same).
I’ve had two 5S bend on their own. I’m not sure if it was due to heat or the battery. I would have to dig around but you could clearly see 1 corner lifted enough to make it rock. I’m not sure if was from my old job where I used a plasma cutter frequently, or if it happened on it’s own. The glass didn’t break, and was in a case.