If this sensor works perfectly with 100% accuracy, few would have a problem with it. It won't, so we do!
If implemented, it'll mean some buyers will take their pristine device back to a store, and be wrongly told (thanks to these sensors, very firmly told) their warranty is invalid.
I'm sure it would also invalidate a lot of silly/fraudulent claims too, but not being a scammer they're not the cases I'm worried about.
Personally, I'd prefer Apple focused on making their devices more durable, rather than trying to welch on customers' warranties.
Well, it seems you're assuming a worst case scenario that Apple will determine whether or not to cover a product under warranty based entirely upon what has been recorded by sensors.
I think they will continue to evaluate units on a case-by-case basis and unless they see evidence of either: a.) serious abuse (i.e. unit submerged in liquid, visible corrosion on circuit boards, or cracked display) or b.) continuous, frequent abuse (i.e. unit being thrown around, dropped on numerous occasions, serious dents), they will continue to provide coverage.
It just provides Apple with an additional layer of protection from customer abuse. I frequently see Apple cover products damaged by obvious customer abuse now, and I don't see why that would change with these sensors. I think they would more likely be used to determine why a customer who has had two optical drives replaced, or two hard drives replaced, needs a 3rd one within a year.
It would enable Apple to say: "Look, this unit has clearly been dropped and this likely caused the problems you are experiencing with your optical drive. We will replace it under warranty this time, but if this part fails again and your sensors show additional physical abuse, we will have to deny coverage."
If Apple does end up using such sensors as an end-all determination for warranty coverage, I agree you'd have every reason to be upset. I just don't see it happening, as someone who works with Apple on warranty jobs everyday.