If you only need to swap it out once every few years it seems like a huge waste to have a removable plastic door and separate housing taking up space. It's not too difficult to swap an iPhone battery yourself. iFixit has detailed instructions, but there's a bunch of mall kiosks or small shops that will do it for you in an hour and around me they only charge around a $30 premium ($110 total) more than the cost of the battery ($89) (I was comparing an iPhone Xs).

With the iPhone 4 it was literally removing 3 (sadly proprietary) screws and something like a guitar pick to safely pry it out. Those replacement batteries sell for $11. With newer iPhones, because of waterproofing there are more sealing adhesives and to make room for the stretch-tape you might have to pop off a few components.

No, the goal is to swap them out during the day.

When I'm seriously on-the-move, and the phone is critical to the mission, and I'm not able to be attached to a cord or carry a bulky external battery bank - a few thin, flat lithium batteries are an ideal solution. I'm talking about situations where high availability is key. Sometimes I bring 2 or 3 backup phones just in case, all the same model, and they all take the same batteries. It wasn't uncommon for me to have about 8 spare batteries on some trips. Half the batteries were always charging at a base station, and I would swap those out in a few seconds and get back out where I needed to be. Yeah it's not everyone's situation, but now I just need to have 4 or 5 devices instead, and have to use a stupid external charger which never does what I need it to. Operationally it's made me suffer having to use modern devices without removable batteries.

Charging from an external battery bank is just not ideal, it's inefficient, and bulky, and increases the heat of the device while charging which can actually be a problem in some situations.

Changing batteries takes less than a minute, creates no additional heat, and restores the device to 100% instantly.

I did this with every phone I had before they took the option away from flagship phones.

Getting old boots resoled is probably a more apt analogy here. Soles wear down, take them to a cobbler—good as new. I’ve got dress shoes older than my teenager and they still look fantastic. My current daily wear boots are 5+ years old and have had new soles a few times. Sadly there’s too few good cobblers left.