You have two options. Make a phone that can easily opened up to change the battery, or make a phone that is somewhat or completely waterproof. You can't have a phone that can easily be opened and waterproof. (Maybe you could but nobody want's to design it.)

Phones used to have replaceable batteries but people complained about their phones not being waterproof. So manufacturers started sealing them tighter meaning no replaceable batteries.

Batteries CAN'T last forever, so if you have a phone that can't be replaced you will have to get a new one at some point even if you don't want to.

Although if you are willing to put forth some effort you can replace a battery even if it's not suggested. I have an Axon 7 and the battery is not replaceable. Yet I bought a battery off ebay and cracked open my phone and replaced the battery and it's now like new. Stick it to the man if you are able!!

Didn’t Apple get caught up in a designed obsolescence thing involving the batteries?

No, just using cheap batteries, probably from Dollar Tree. LOL

Yes what you will also notice s pay as you go phones rarely work for the month of december making sure you pop into the shop to buy a new one or get talked into signing a contract

I think so. Caleb had an iPhone 5 and it was impossible to keep charged. I eventually upgraded him to an iPhone six and it’s much much better. As a disclaimer, These are used phones I’m buying from individuals on a local yard sale site.

All brands,or specific one(s)? I've had a Samsung Galaxy S5 for many years(5 or so),recenly upgraded to the Samsung Gaxy S9,world's of difference,keeping the S5 as a back up if needed. Replaced the S5's battery not long ago,no problems.

Yep.