If your only device is a desktop computer and you don't have a printer how are you supposed to take your ticket with you?
On your phone. I doubt there are that many people who have a desktop computer AND have no printer AND don't have a mobile phone AND travel on the Sleeper AND don't know anyone with a printer.
Having a computer without a printer (or not even having a PC and just doing everything on your phone and tablet) is a millennial thing. There basically aren't any millennials who don't have a smartphone.
Having no smartphone is generally an older person thing. Older people tend to still like to write "print" letters so almost certainly have a printer to go with their traditional desktop PC to print them out.
And if they don't have Internet access, they aren't going to be purchasing tickets from a website, so that's out too!
And relying on a phone not to crash or run out of charge is fraught with risk.
Get a battery pack, they're about 20 quid and will ensure you do not run out of battery (most around that sort of price point can charge a typical smartphone fully about 3 times). And we aren't talking complex apps, we're talking PDFs. If your phone routinely crashes displaying PDFs, you probably need a new one as it's faulty.
That said, I do think it would be good if TVMs could, on entry of some details, reprint e-tickets to avoid this issue. I think there are operators worldwide who do that.