Asus tried making a phone-tablet convertible way back with their Transformer series, Windows Phone had a feature where you could plug into a dock and use it as a full-Windows workstation (kind of), and in 2018, Razer showed off their concept for a phone-laptop convertible. As far as I know, Asus gave up due to poor reception and poor sales, Windows Phone went the way of... Windows Phone, and Razer never made a mass market device from their venture. Even now, Pine64's PinePhone has an upgraded convergence edition, continuing the concept of a universal Linux distro for phones, tablets, and desktops that Ubuntu started.
There are a lot of benefits to this, one is that assuming you had a phone, laptop and desktop as separate devices, this can save a lot of hardware, especially since in most cases, you're only using one device at a time. Lots of people already use their laptops as desktop replacements, so if a phone-laptop convertible can match that, then it can easily become someone's only device. There's an environmental benefit too, phones are much more power efficient per unit computation power, and since you don't need to upgrade the laptop chassis or your desktop peripherals nearly as often as the active compute hardware, you'd be producing less e-waste. I personally also really like the idea of having one computing device to rule them all.
Do you think this concept is straight up dead in the water, or, as phones become more powerful (especially since laptop chips like the Qualcomm SQ1, Apple M1, and the upcoming Intel Lakefield can already be passively cooled and still be quite powerful), operating systems improve to support seamless transition between device modes (Some Linux distros are working hard on this, and Google's Fuschia OS is planned to replace both Chrome OS and Android in the future), and with the USB C and Thunderbolt standards offering fast I/O, it will actually gain the mass adoption in the future that it isn't able to achieve right now?