Who says it has to fully replace a laptop for most people? It would be nice to have in a pinch though. The way things are going, each desk in offices and eventually homes is going to have a monitor or dock where the device plugs in via USB-C/thunderbolt, and being able to fall back on plugging in a phone or iPad in a pinch would be great. I wouldn't necessarily do it every day, but the option would be cool.
99%+ desks at the institution I work at have a USB-C dock+multi monitor setup already, where the staff plug in their laptop.
Also, you'd be surprised how quickly some organisations would want to save money by not providing laptops. And there's also the fact you could use it if your laptop is broken - in business settings laptops break all the time and having to source a spare for the 2-3 days it's going to take to repair that is a constant annoyance. Having multiple devices be capable of replacing a desktop for standard work even if it's just tiding you over for a bit is cool even if it doesn't mean you throw your laptop away tomorrow.
Whether it's a phone or something else isn't necessarily the thing that interests me though. What's annoying is in this new age of Apple silicon, the lightest and smallest device Apple sells that runs macOS is the 13" 2.7lbs MacBook Air. They used to sell a 12" 2lb laptop, and I just really wish they would sell something in that weight and size class again.
Agree, it's "nice to have in a pinch". I have used Dex a couple of times over the last couple of years.
As for 99% in your institution etc, as I said it doesn't matter most of the time, as there is already a more capable OS available.
And as a laptop replacement on the go, you need a laptop dock, which more "inconvenient" than an actual laptop.
As for replacing a broken laptop, yes, it's part of "in a pinch" and it's still a partial replacement.
The issue I have is when someone says "I could replace a 3.5 pounds laptop" with a phone. No, you would still need additional hardware on the go and on site you probably already have desktops available. And above all, there are all the missing software and features from a desktop OS.
And for the last point, I am the first person to ask for a 2lb MacBook. I refuse to buy any Apple Silicon MacBook over 1kg. I have 2 12" Macbooks and I still use them daily. Plus a 2lb cellular Thinkpad. So I am the first fan of ultraportables.
Dex or Stage Manager in a phone are no replacement for that.