AIUI, the main problem in the cell phone era is that by the time you create a notebook shell with an even halfway-decent screen, keyboard, battery, and the other things you'd want in your shell, it's hard to sell it next to the thing right next to it that is all that, but they also stuck a cheap computer in it (and is therefore no longer a dock). Yeah, it's $50 more expensive, but it looks way more than $50 more useful.
What may shift the balance is that slowly but surely USB-C docks are becoming more common, on their own terms, not related to cell phones. At some point we may pass a critical threshold where there's enough of them that selling a phone that can just natively use any USB-C dock you've got lying around becomes a sufficient distinguishing feature that people start looking for it. Even just treating it as a bonus would be a start.
I've got two docks in my house now; one a big powerful one to run the work-provided laptop in a more remote-work-friendly form factor, and fairly cheap one to turn my Steam deck into a halfway-decent Switch competitor (though "halfway-decent" and no more; it's definitely more finicky). We really ought to be getting to the point that a cell phone with a docked monitor, keyboard, & mouse for dorm room usage (replacing the desktop, TV, and if whoever pulls this off plays their cards right, the gaming console(s)) should start looking appealing to college students pretty soon here. The docks themselves are rapidly commoditizing if they aren't there already.
Once it becomes a feature that we increasingly start to just expect on our phones, then maybe the "notebook-like" case for a cell phone starts to look more appealing as an accessory. We've pretty much demonstrated it can't carry itself as its own product.
That would probably start the clock on the "notebook" as its own distinct product, though it would take years for them to finally be turned into nothing but shells for cell phones + a high-end, expensive performance-focused line that is itself more-or-less the replacement for desktops, which would themselves only be necessary for high-end graphics or places where you need tons and tons of storage and you don't want 10 USB-C drives flopping around separately.