Sure, and so are the motherboard, screen, digitizer, etc. But that doesn't make it consumer-friendly. The battery in my phone is also user replaceable. A new battery costs almost $20, and requires me to pull the back cover off the phone with my fingernails. Whatever excuses a company makes for not offering that feature really just boils down to making money.
I'll take removing two screws to access the internals over having the entire thing fly apart when it's dropped on the ground any day. Do you remember removable batteries and cases from the early Nokia days? Even a minor jolt could interrupt the power supply and freeze the phone, so even if it didn't explode into four pieces, you still had to disassemble it and re-insert the battery.
Solid construction and mechanical fasteners are a feature. Servicing any part of an iPhone is not that hard; lack of serviceability is a myth.
Additionally, batteries are not the most commonly replaced part. So far my SE has had four screens and a lightning connector. I'll do the battery myself when I notice performance degrade, too.
It is user replaceable. You can buy the part and the tools and do it yourself if you're handy with a screwdriver. It's not a simple matter of snap-out/snap-in, but it's not much harder than replacing a car's windshield wipers, and much easier than something like replacing tires or brake pads.
I've done this on many phones. The phone often isn't the same as it was initially because it was not designed for this. Many phones have tape that has been adhered through the use of heat. Changing the battery requires removal of many screws and disassembling very sensitive parts. Little mistakes can make the phone inoperable.
Not to mention you take it to a technician who has the tools and you'd assume is doing a good job right? No, often it's some high school kid who is not invested in doing a good job but rather just getting it done. It's sloppier than if you were to do it yourself. So you have to resort to buying hundreds of dollars of tools and taking painstaking hours to get it done right and even then it might not turn out.
To the other posters who argue'd that we make a choice about this - no we really don't get a choice. If you want a cutting edge phone there are simply 0 options with replaceable batteries. This is planned obsolescence in an age where my several year old Motorola works just fine for my needs.
Certain phones can be done with a cheap kit. Some require heat guns (for certain adhesive tape) and more advanced equipment. Plus if you don't want to take any risk you really should have an anti-static environment with proper grounding.
The car is a good analogy.. As @mikeash said, it is replaceable, you just need tools, parts and expertise (and time and willingness). Same as replacing parts on your car.
It is just as user replaceable as those car parts, if not more so. It requires only a few cheap tools, some time, and maybe watching a YouTube video example before starting. I'm generally more comfortable dissembling my phone than my car since the consequences of messing up are much smaller.
It's a "regular maintenance" type thing - less common than an oil change, sure, but perhaps just as necessary. So why not make it as easy as an oil change?
Having done both, I can tell you that replacing the battery on an iPhone is already easier than an oil change on a typical car. The iPhone parts are expensive, but it's a relatively simple operation for a trained person.
Not to mention that an Apple battery replacement is about the same cost as having someone change the oil in your car if you use full synthetic. I just don't get the complaints. If the battery is dying, get a new one. If you don't want to spend the money, then Apple has provided a feature that lets your phone keep running.
Not to mention there are an infinite number of 5 minute tutorials on YouTube. You don't have to be trained, you just need a set of eyes and a brain. I'm glad an increasing number of phone manufacturers, not just Apple, put form factor and durability above user laziness.
Cars, you may have noticed, are big. Apple has made a tradeoff between replaceable batteries and small phones, and the market has more or less agreed with this decision.
One of the reasons I upgraded to an iPhone X was that my 6 Plus was getting slow. I'm just now finding out that I could have replaced the battery for $85. Am I mad!?
No, I'm not. 3 years is long enough for a smart phone, I probably would have replaced it anyways. And I like the X way more.
My guess is most people plan to replace their phones after 2 years, and few have this problem before that. So disclosing probably doesn't change more than a minute number of purchase decisions. Though I don't doubt that some marketing person at Apple influenced limiting disclosure of this fix because they thought it helped upgrade rates. If that person exists, they should be fired, because it damaged Apple's credibility.
Instead of a hard to find value in settings that tells you your battery needs replacement, they should have a one time warning when it's first detected that explains this behavior.
Thank you for the your very informative message. I think messages like yours are somewhat lacking in this discussion.
I understand your argument. I think I would be somewhat mad, or maybe it's better to say, disappointed. While I most likely would have bought a new phone anyway, I would despise that Apple took away my ability to make this call. It also means that I could not refurbish the phone and pass it on to parents/friends.
In the last years, I am somewhat disillusioned by Apple. Instead of engineering a better overall package, the innovations seem to be based on cheats (I would the /undisclosed/ cpu down-clocking as such), cutting corners in QC and questionable design decisions (touchbar - why not with some touch feedback? No headphone jack for reasons already disproved by android phones.)
I didn't buy in into the ios eco-system, but like the mbp and mac os. But the number of advantages get smaller, and the number of disadvantages grows.
I mostly agree although I don't really know if my iPhone 6 seemingly getting slower is due to power throttling or general software optimized for more performance over time. The geekbench numbers are still fine. I'd hesitate to get a new battery purely on speculation my phone would get faster. So it would be nice if there were some indication of where the battery is in its life. Though like you, I'm upgrading to an X in any case.