Even smartphones hit a point of diminishing returns after a while. A 5 years old Samsung Galaxy S8 would be still perfectly usable, and better than most cheap devices today, it's just unsupported, because go buy the new one. Fortunately you can slap a ROM on it and keep using it, you can even replace the battery. Unfortunately millions of these devices are already chilling on a landfill somewhere. It's not like we are running out of the resources to make new ones, or there is an environmental collapse looming around.

Even if the processor continued to work, the flash storage and battery in these devices aren't going to keep working forever. If we want to have longer lived devices, these parts will need to be standardised and user-replaceable

I think a thicker phone would exist if the feature and performance enhancements were really needed. I would have to conclude that making them thinner is the only real enhancement we've seen. Every other possible feature is traded away for it. The battery is no bigger, you cant get a headphone jack, you get just one port, no cooler, all because it would add mm's.

Do we need cameras this good? The resolution is fit for a huge display - that no one uses anymore. You would have to go back many generations to find one that takes pictures that are not good enough.