would it be possible to live in the modern world only using a laptop?? no cellphone anything

You would still need a internet source, but yeah. The hardest part would be getting a virtual phone number that qualifies for receiving account text messages when necessary. Or using apps that inherently tie themselves to a phone number.

You're going to look like a doofus holding your laptop to your ear, but it's totally doable.

A person can live without any kind of computer at all. Believe it or not, there used to be a time when computers and smartphones did not exist and people still lived. Now, that doesn't mean it will be easy.

Jobs usually provide them when they are necessary to your job, and public libraries usually have computers that are free to use for things like schoolwork, job application, and other such tasks.

Yes, it is possible, though fairly prohibitive. You can get modules for connecting a Sim card to your laptop to make and receive calls and SMS, so all the third party authentication messages and calls can be done. Apps can often be run through an emulation layer for Android, not so much for iOS, but yeah, it can mostly be done.

That said, there are devices like the Pinephone which run a mainline Linux distribution like Arch or Alpine and can also do calls, SMS, and if you are brave use some android apps. If you can use a Pinephone for it then you can use another Linux laptop with a phone module to handle everything else.

I am much more worried about things like banking now. A bank that allows use of an app on your phone with NFC is great but when they take away using anything else I would become a little more worried. If they can keep an option open, say having standard cards along side phone NFC, then the lock in is much less profound. If we get to a point where they cannot operate without Google or Apple giving the go ahead then they cannot move away if things get bad, so those monopolistic companies can extract more value from those banks and that gets passed down. This applies to tonnes of other services like taxis, food delivery, booking doctors appointments, and so on.

There'd be a few times it would be really inconvenient.

For example some concerts only allow tickets from the Ticketmaster app, not even a printed ticket. Though possibly showing the ticket on the laptop would work but then I'm not sure they'd let you bring a laptop into a concert.

Probably yes, unless you are using any services that require authentication tokens to be sent via SMS/MMS. I also haven't used any authenticator software on my laptop, but I know that they at least exist in the form of Authy, for example.

Personally I prefer to have 2 authorized devices in case one or the other fails, but admittedly the risk is rather low and it's more a thing of convenience. I'd manage to get to the nearest store and have my system restored from backups within a few hours regardless.

Traveling is going to suck.

If you are driving places, you are going to fall back to using a map or maybe a GPS device if they still sell them. If you are going to places with paid on-street parking, paying may be more difficult as cities have started shifting to a mobile way to pay, so bring your laptop.

A lot of rail and airline ticketing is now done through phones, with some airlines making you pay for a printed ticket. And if anything goes wrong, these companies have pushed a lot of customer service to mobile.

If you don't have a car, that means your best luck is a taxi. That might work in some cities, but don't expect it to be as fast or convenient. Also load up on mass transit schedules, since a lot of transit agencies have gone to an online only posting of schedules.

You don't even need the laptop but it would definitely be helpful: If you need to call anyone's phone number, just use a VOIP service and you're good.

I think thank pads still have the option to take sim cards so yes