There are still plenty of example use cases which aren't covered by the existence of QR codes and phone scanning. Documents which need ink signatures. Boarding pass backups so you don't hit disaster when you are running late for the flight and your battery dies. Government agencies which insist on doing things by post. Sending things to non-tech capable relatives or customers who don't know what to do with a PDF. Leaflets and handouts for community meetings. Notices which have to be physically displayed in a building or on a window. Homework assignments for young children.
Some of these you might be able to workaround with some added inconvenience (e.g. carry a spare phone battery). Others are simply impossible. I'm not going to fail to buy a house because I can't comply with the mortgage company's requirement to return a signed deed for example. Much as I rarely use my printer and wouldn't be bothered at all if it stopped being required, I accept that I need to be pragmatic about it.
For people who have an office or home where a printer won't take up valuable space or look out of place, use it semi-frequently (let's say a few times per week or month), and might have to deal with the occasional urgent case, owning a $100 printer is a small price to pay to avoid having to spend an hour visiting the library (depending on distance of course) every time you need to perform 30 seconds worth of printing.
For a bit more money, you can get a multi-function printer with a built in duplex scanner and document feeder, which actually helps with running a paper-free environment. I have one of those sitting on my desk. Every piece of mail that arrives goes first into the scanner and then into the recycling bin, and avoiding trips to the library for printing is an added bonus.
Bording passes are are almost always provided for you on check in for international flights (all you really need is your passport). And honestly I can't recall my phone battery ever reaching 0% - then again, battery lifetime is one of my primary criteria when choosing a new phone. There is still some risk that the phone blows up (literally or figuratively), but that's getting into theoretical territory.
I do have a printer for convenience but it's rare that I actually need it and I have lived for long stretches without one.
I never used paper for anything else, so from my point of view, it's not on the decline. But just steady and stable.
Short story you're trying to read but the endless scroll makes you lose your place? Print it. Sheet music you want on-the-go without buying an ipad pro? print it. Funny picture you want to tape to your wall? Print it.
I think nearly everyone on HN is aware of and may use QR code tickets sometimes but that's irrelevant to the broader need for printers in business and government.
Things I hadn't immediately thought of as an engineer. Good point!
Until you need an app that also monitors your neighbors heartbeat just to be able to enter the venue. No thanks I'll keep printing.