The batteries are replaceable, just not (without a few tools and knowledge) end-user replaceable.
And even if they did make the batteries user replaceable, that is of pretty questionable value for the consumer. First, a lot of people are going to buy $5 replacement batteries on Ali Express and their phone is going to get bricked or catch fire. Related to that, most people are unaware of how dangerous handling loose lithium batteries can be.
Second, it makes the phones much more vulnerable to liquid damage, as well as much physically larger.
Except user replaceable batteries were the norm until recently, and we did not have the situation that you describe. More importantly, user replaceable batteries also allows mobile phone stores to offer dirt cheap replacement services, equivalent to jewelry stores offering to replace watch batteries.
Those replaceable batteries were also much lower power (in terms of capacity and max current) than the batteries of today. My iPhone today has roughly the equivalent of an 18650, compared to the Razr I had 15 years ago, which has roughly the equivalent of a AA.
For sure, I would love it if my phone had replaceable batteries. I carry a couple 18650s and 16340s in my backpack for my flashlights. But there are a ton of downsides to having them, particularly in a smartphone.
That seems much the same argument as saying "If you want a phone that won't be slowed down by an updated, buy a different phone." Such an argument has been tried and rejected.
Lack of ability to replace the battery seems to suggest otherwise.
>There is no magic solution here.
Replaceable battery with a message indicating when battery health is low enough it needs to be replaced seems close enough to a magic fix.