As much as I hate most of the ideas the EU comes up with, and believe me, some are really crazy, I infer this is actually a very good idea. Both of my last 2 handsets, once with battery life close to 4000 mAh , the other one with 5000 mAh, started losing their battery life very fast after about one year of use, despite using (in moderation) various technologies, battery care and what a not which should help battery life.
Now, I don't mind paying for a new battery, would have done it in a heartbeat, if only there was a possibility to change it myself. I can change battery now as well, of course, but I pay to the middle guy almost as much as the price of the actual battery.
Having batteries not readily replaceable by the user might make waterproofing easier, but definitely not impossible.
My last handset with an easily replaceable battery was Asus ZenFone 2 Laser. Even then, 2015 or 2016 probably, can't remember, handsets with user replaceable batteries were quite scarce and I had to look long and hard in order to find one that would also suit my other needs. Nowadays, the handset is still rocking perfectly fine with its new, about a half year old battery and went to the grandmother, who is very happy with it.