The once revolutionary mobile phone industry is in complete stagnation

Smart phone advancements peaked for me at least 10 years ago. At that point, one could buy (what I would consider to be) a solid phone with "good enough" specs that work perfectly fine for $200ish. Ever since then, the only reason I feel compelled to buy a new phone is when my battery can no longer last the day (around 4 years).
Less innovation, less reason to upgrade, less waste. What's not to like?

I'm one my OnePlus 6 and everything works just as it is supposed to. :p
There's little justification to refresh traditional phone hardware annually now. I'd love to see:

1) Biennial or triennial hardware updates that are a major update and reduce price pressure. It's nuts the number of people juggling their finances / available credit just to 'afford' a 'new' phone that adds a vanishingly small amount of marginal utility over and above their existing (not even 'paid off') device. Update 'Pro' models this year and 'non-Pro' models next year or whatever keeps the marketing department busy.

2) Focus on better streamlining IOS and Android. Give me a (user-friendly and supported) way to hide, disable or remove features and default apps cluttering up the UI and bloating the OS that are rarely (if ever) used.
Another word for "stagnant" in the world of products is "maturity". One the number of vendors is winnowed down from where it was years ago, there is little reason for any of the major players (Apple, Samsung, Lenovo's Motorola, others) to innovate. And, what other apps remain to be done?
Phones are presently stuck at a single form factor.

I see future phones evolving into a two component system. A core watchlike device that houses the computer and radio components, and a separate display screen. The display screen would be ultra thin, lightweight, and foldable. The folded state would be very small, possibly the size of a pack of gum. Unfolded it could potentially be the size of standard notebook paper, or even larger. The folded screen would snap onto the core unit. The entire phone could be carried on the wrist, neck, or possibly as a necklace. To use it you would unsnap the screen from the core unit, then press press a button on the screen or flick your wrist. It would unspool to full size.

There are other evolutionary paths but I think this one is most likely for the near future.
Partially the market is stagnant because the big vendors will not try anything to REALLY stand out. (Well, I'll admit foldables are something new, but other than that.)

Keyboard phones? When TCL make the BlackBerry KeyOne, then the Key2 came out, they sold as fast as they could make them. They said they didn't hit the sales they wanted, but they had people wait weeks, find out the new batch sold out in like a day, then say "the hell with it" and buy something else. I love my KeyOne and am pleased that at least I can order some keyboard phone from UK and pop my SIM into it if my KeyOne wears out (I LOVE having a keyboard to tap things out on!) I'm sure the likes of Samsung or LG or the like could manage to produce the phones fast enough to not have people be turned away due to slow production.

Sony made those phones with a Dpad on it and you could play Playstation games on it. I don't think you'd want to try to play like PS4/PS5 games on the phone (if a phone had enough power, it'd probably run pretty hot!!!) but if you had something that'd play Switch games or the like it'd be pretty unique and likely be a seller.

Just making a phone where the battery & screen are easily replaceable would make a phone stand out compared to the rest -- of course the phone companies would be nervous that if someone's phone is too repairable they won't buy a new one. Most people still find their phone is getting "long in the tooth" after 5 years or something anyway, I'm sure people would still replace the phone even if they don't have a cracked screen and dead battery pushing them into it.

The vendors all go for making a slab, as big a screen as they can fit, as thin as possible, so they end up making what appear to all be the same phone, then wonder why their phones won't stand out in the market....
Windows 10 Mobile was a cool operating system, but everyone dumped it for Android and iOS. Now the market is stagnant, who would have thought…
Windows is a terrible mobile OS. I have a handheld gaming machine that runs Windows. It runs games just fine, but the interface is painful. It is definitely not fat-finger friendly. On an 8-inch screen everything Windows is small. iOS/iPadOS is far more optimized for mobile use than Windows.
Another word for "stagnant" in the world of products is "maturity". One the number of vendors is winnowed down from where it was years ago, there is little reason for any of the major players (Apple, Samsung, Lenovo's Motorola, others) to innovate. And, what other apps remain to be done?
Exactly, I don't know how much more we can squeeze out of a mobile phone. Sure, we can make them faster, foldable, easier and faster to charge, but what major functionality needs to be in a phone, that's not already there today? It's kind of like TVs. Really, other than the display tech, what has changed in terms of TV? Nothing in the past half century. We still get movies and shows on a screen. The source may be different, but the content is still the same thing.
"When even market leader Apple struggles to come up with new compelling features"
When did they ever come up with a new feature?!?