and for older people who often do not use mobile phones ?
The only purpose the QR system serves in booking a seat you're already sitting in is to let the digital reservation system know when that seat will become available for the next reservation.
If the OAP sits in the seat without a reservation, the digital reservation system, thanks to the sensors it uses, will block the seat off from reservation until it has been empty for a pre-determined period of time.
If you take an Aberdeen bound service as an example, if you get on at York and scan the QR code telling the system the seat you're in will be occupied until Edinburgh, the reservation system will make that seat available at Edinburgh for a new passenger to reserve in advance.
If a little old lady gets on and sits next to you, without scanning the QR code, the reservation system will record the seat as being unavailable, and it'll check all the way north whether the seat is still occupied. The little old lady gets off at Newcastle, and nobody sits next to you, by Berwick, the digital reservation system will have determined the passenger has either moved or has alighted at Newcastle, they no longer want that seat and it'll be released back to be made available for a booking.
It might be the case, however, that the seat isn't released back for reservation in time for someone boarding at Berwick to book it electronically in advance, but they'll still be able to sit down in the seat. They could then scan the QR code and say they're going to Edinburgh, making the pair of seats reservable from Edinburgh onwards, or they might not, and just sit in the seat, in which case it might take the reservation system until Inverkeithing before it releases the seat back and makes it reservable.
It's a little bit complex, but it does mean you'll never be left sitting on a seat that is reserved whilst you're sitting on it.