If you're brave it looks like you can replace it in only 5 steps. The caveat being that those 5 steps look very difficult, and in some cases require special tools.
I didn't downvote your top comment, and miss phones with actual keyboards, but Google Pixel and Samsung Galaxy aren't the only phone brands out there. 5" devices still exist. Devices with replaceable batteries still exist. Devices you easily can install custom ROMs on to not rely on the vendor for updates still exist. It's quite possible the combination you like isn't out there, but it's not total monoculture yet.
Samsung hardware + custom ROMs are a path I took in the past (with the S4 Active, which by the way was waterproof and had a replaceable battery, but somehow never achieved tremendous popularity).
But yeah, it's something worth thinking about. The major appeal of a phone like the Pixel is less the hardware than the vanilla aspect of the Android OS.
All another hardware vendor has to do is attempt to stay up to date, and minimize their own bloatware if that's a bottleneck in their time allocation allotment. I'm surprised no one sees that a lot of Android users clamor for a vanilla, up-to-date Android OS phone.