I went to a convention once, deciding that it was redundant to bring my laptop with me when my smartphone alone should have been perfectly capable to do all of my online only Internet tasks.

I ended up having my credit card skimmed from an ATM, and someone withdrew around $500-1000 as a result of that, which was about half of the money that I had on my account, and allocated towards that particular trip.

Let me tell you, it was not comforting, sitting on a hotel bed with a tiny overheating smartphone (Galaxy S2 back then), navigating the bank's website with my thumbs, on a slow mobile connection, which kept on crashing. Their phone lines were busy for 3+ hours because it was a major event.

I realized in that particular moment, that I always needed a backup, high speed computer (or laptop) with a full physical keyboard and mouse for specifically those circumstances, where time was absolutely critical.

Mobile-only people are how social media exploded. Not everyone could afford, or needed, a computer. Their phone was a gateway to the internet.

I'm a power user, so I need to view a full screen and get desktop websites. I need the speed of a keyboard and mouse.

The disparity between desktop webpages and mobile apps/webpages is too big for my uses.

I need the multitasking and customizability of a computer.

I'm actually trying to go more desktop and less smart phone.

I know of a mobile-only 5 person household, but I need a PC being that I'm a software developer and for a handful of indie games.

Mobile is great for consuming. It’s not as great for creating.

More consuming and less creating sounds like a good way to slow progress.

The obvious next leap is full-on Idiocracy.

Back in high school I tried using my first android smartphone for studies and notes when my laptop had broken down. I connected a (wired) keyboard and mouse, but it got tedious really quick. I guess one can theoretically work off only a phone, but even simple tasks take considerately longer to perform. The only real positive thing was the small carry size and the addition of a touch screen.

Having learned nothing from the first attempt, I later brought only a phone to surveying field work as newly graduated UAS robotics engineer (needing it for route planning, weather stats, auxiliary vehicle integrations, data transfers, etc.), and, well, it could do everything I asked it to do, but it would again have been significantly faster with a proper mobile workstation. Since this was with a phone without USB-C (so no PD hubs) the additional power drain also meant that the phone barely lasted throughout the day, even if I connected it to a power bank every time I did not need it. It also kept overheating in the mid spring sun. I knew that smartphones are bad at heat management, but I hadn't expected it to be that bad.

At last, I've recently had half a year where I've used only my phone and a Steam Deck for all my computer related tasks (between jobs, rig was in storage). The SD allowed me to do everything from gaming to light dev work, and the phone was enough for all communication (bank, contacts, E-Mail, etc.) and social network requirements. I guess I could live that lifestyle again for a limited time, but having back my rig is just so much more comfortable, especially for heavier dev tasks.

TL:DR: One can definitely live with only a phone, especially if the lifestyle is less technically inclined, and even a techie can ultimately do most tasks on a phone, but while a properly configured phone can do most tech tasks it's just so much faster to have proper computer at hand.

It would be neat, but 90% of stuff absolutely sucks on mobile. Trying to SSH into a server and transfer files to fix something, banking websites, websites in general, and so on..

My PC has been with me for like 10 years and it's working great. If anything goes wrong with it, likelihood is that I can just replace one component and get working again soon.
All it takes is one slappy toddler or someone knocking into you at the wrong moment, and that 1300 dollar device is not worth the cost of repair and destined for a landfill.

I personally like having purpose-specific machines. I don't want to associate my mobile phone with work and I don't want it going off with every work email I get. I also want to be able to work faster than I would with a tiny display and no real keyboard.

At the same time, I want to use my work machines as little as possible, because that means I am working as little as possible lol. I find that physical separation of professional and personal equipment, beyond aiding workflow and efficiency, is psychologically helpful for me destressing when I'm not working.

Plus there are so many things that, sure, a phone can do, but a larger device is just so much better at. Writing anything over 200 characters, watching videos (particularly watching videos, erm, one-handed...), reading and writing anything complex, running multiple programs/tabs, accessing complex sites and apps, and a whole lot more.

Do I use full-sized devices much less because of my phone? For sure. Could I ever imagine going phone-only? Not a chance.