Re: Done their work well?
Identify a problem that could be solved by a pocket-sized lump of sensors and processing power
That's it exactly. There was a period when phones were adding new features like cameras, full browsers, GPS, MEMS, touch/face ID. They've replaced cameras, camcorders, GPS / in car nav units, music players, dictaphone/voice recorders, pager, photo album, alarm clock, calculator, calender/planner, notepad, barcode scanner, measuring tape, even stuff like flashlight, level, car keys, and wallet in some cases. We've pretty much run out of stuff for smartphones to replace!
Phone cameras are incredible now, a decade ago I could blame the quality of my pictures on the phone's crappy camera. Now the problem is 100% my fault, as evidenced by all the amazing stuff you see on Apple's #ShotOniPhone ads. I'm sure they will be further improved, but it makes less and less difference except to a tiny segment of the population who are good enough photographers that they can fully exploit the new features.
There are little niche things added to some phones that a few people care about, but not enough for them to be widespread, like IR remote, FLIR camera, and so forth. Those will never become standard because the cost/benefit isn't there (well if they could add a FLIR for a buck or two it would probably be worth it even though people don't often need one, but getting the price that low is sort of a chicken and egg situation)