I always wait until the gate is closed before tapping my card on the barrier.
Definitely not a good idea in London especially at peak commuter hour
Whereas if I had an e-ticket I would have to switch my phone on and wait a couple of minutes for it to boot up, then try to get an internet connection, log onto the website I booked it on, download the ticket, then try to find where my phone has stored it and hope that the file hasn’t got corrupted as pdf files often tend to do.
Normal people don't keep their phones switched off, normal phones don't take a couple of minutes to boot up, and normal passengers will have stored their tickets on their phones. On my phone, a ticket that is to be used currently or in the near future will even become an option to pop open on the lock screen without needing to unlock the phone.
In short, you're not describing an actual scenario.
Cheque guarantee cards were a stupid and totally unnecessary idea. The card could simply have been used to process a card payment using a manual machine instead, exactly how credit cards were processed for many years.
They were good for paying on Wednesday when your account was overdrawn and your salary was going to credit on Friday
Also, debit cards only became generally available in the mid to late 90s.
Especially at barriers, its probably in sleep mode as you approach, need to wake it up, start the reverent app (TOC, email, browser, whatever), find the relevant barcode (may need scrolling if its in an email), and do all this one handed while approaching the barrier while carrying another bag. If you don’t do it soon enough, you clog up the barrier.
Also unrealistic; many phones will have a link on the lock screen to pop up the ticket, scan it, and you're done.
All that said, however, I rarely use e-tickets. The reason for this is that I'm resident in Southeastern land.