I think this factor is more relevant today than five years ago. At least my upgrade cycle has slowed down, and I'm often subjected to poor battery life before the apps get too demanding for the hardware itself. Really frustrating with non-user serviceable batteries when this happens. How a phone feels like new with a fresh new battery, that "oh my god, how..." feeling with the battery life, just cannot be underestimated.

I agree - and I think most people's upgrade cycle has slowed. Most smart phones from 2012 onwards fulfil basic consumer demands (calls + messages, nav, apps, basic pocket camera).

You are able to get two days of run time on a Nexus 4? I couldn't achieve this even when new.

Replacing the battery on a Nexus 4 is a trivial affair(I am not saying this to take away from your non removal battery point but letting you know its pretty easy).

Two days of battery life was while using Sailfish rather than Android. I'd imagine part of that is due to the fact that Sailfish doesn't have drivers for the GPS chip in the Nexus 4, and therefore would never power it up.

Aside from that, I still get decent battery life out of it even in Android by carefully policing what runs on it. I have very few applications on my phone that run in the background, and I don't have the Google apps installed - I use microG instead, which is a FOSS implementation of (a subset of) Play Services.

I'm aware that the vast majority of users probably wouldn't like using their phone the way I do. That said, this should make it clear that battery life is very much affected by the software you run.

Even if you upgrade your phone every three years, say, someone else will use it after that. Sometime back, I sold a phone that was 5½ years old when I sold it.

If phones had better battery life out of the box, it will help their current owner and the next owner.

Well, if you're talking about giving an old phone a new lease on life with a replacement battery as a once-in-a-lifetime service operation, it's fine to break out the tools to replace the battery.

If you do battery swaps as a nightly workflow so you can have a day battery and a night battery, then yeah pop-out replacement is the only option

Having disassembled the aforementioned Z3C a couple of times, I disagree that it's fine to have to break out the tools for it ever. It's a tough job, with lovely warnings like this one from iFixit:

"Warning first: Never try to move the battery unless it's broken and need to replace. Because there is very very strong adhesive sticker between battery and motherboard.And too much effort or careless operation will break the motherboard."

It's also difficult to maintain the waterproof seal when reassembling the phone. So while the Z3C has great battery life, you're essentially only going to have that for a couple of years.

Funnily enough, for all the bad press Apple gets for glued together devices, the battery is trivially easy to replace in iPhones.

This is, I suspect, a byproduct of the fact that they handle (the vast majority of) their own frontline customer support and repairs. The most commonly-repaired parts are designed to make the process as efficient as possible for the Apple Store techs.

That's exactly it. Apple Retail iPhone 'techs' are relatively low skilled and low paid, and have a very small amount of time to do what they do. The most common parts, like camera, speaker, battery, screen etc. are made like Lego to make it easier and quicker for them to do their job.

The phone isn't quite the same once an unprofessional like me has opened it without guidance. For instance this nexus 5 on which I replaced a cracked LCD now has a little bit of give when I press the back. It used to feel completely solid before.

Yeah, this is because there's a sealing tape that's supposed to go to the frame after opening.

Original frames usually have it attached to the frame, aftermarket ones don't. Some tape or glue needed to close the original one after opening.

While it is of course possible to replace the battery in most phones with specialized tools, for some of the newer models it's very difficult to do without damaging the device. When we're talking about a phone you just bought on eBay with the intention of making it your daily driver, that's a hard sell.

All other things equal, I will always pick the slightly thicker phone with a removable battery over the one that might have me breaking out the spudger.

The specialized tools are a tiny screwdriver and a plastic pry knife on some phones - the adhesive doesn't come in until screen replacement. You can buy battery replacement kits with that stuff included.