First, it seems like you could just make the phone part waterproof and if they dunk it, they have to replace the battery. I have ruined a phone by putting it through the wash, and I would have been very happy to just replace the battery instead of chucking the phone.

Beyond that, I wonder what the reality is for waterproofing needs. Are people taking smartphones diving? IP65 means it's resistant to 'spray' - like rain, not a firehose - and it seems like a battery compartment with a tight-fitting cover would be more than enough for this. IP68 doesn't apparently mandate a depth rating but recent iPhones are tested to 6 meters for 30 minutes. That's more water pressure than 99% of those iPhones are ever going to see.

I don't personally think the non-replaceable battery has anything to do with IP rating, it's because manufacturers want soft-sided batteries with more delicate terminal connections, for volume/capacity reasons.