The iPhone is one of the most popular devices on the planet, and Apple is in a very high profile position because of it. The last time they tried doing anything remotely close to sneaky was the slowing down of phones with older batteries which eventually resulted in a class action lawsuit.
Sure Apple could install backdoor software, crapware or whatever else in an update, but their exposure to a class action lawsuit would be insane. Whatever they did would be found out pretty quick by security researchers and....you have class action on your hands that will probably cost the org $100M - $500M.
Apple isn't in the business of being nefarious, their in the business of selling you an iPhone. It's in their best interest to sell you the most secure and best phone possible, because all they want is to sell you an iPhone.