So whats the real problem here?
the stupid yuppie marketing model by which western rich gits treat new-spec phones as having a one-year use period before sending them on for resale / resuse else where.
Its not a fault with the phones - past experience with computers should be enough to show that anything is potentially recoverable given the right tools. The problem is with the mindset. Anyone with a brain who sends a PC on for resale or scrap shreds the drive - either physically or with a third party electronic tool such as DBAN. Why should a phone be treated any less differently? Especially when solid-state drives are a lot harder to nuke than a "real" hard drive.
The answer is......stop treating phones as disposable fripperies. Keep them and use them until they're knackered and then take the hammer and shredder to them. Get your moneys worth from them, then destroy them. DON'T sell them on