But if you open the app occasionally (eg to show the railcard when asked or just to check the expiry date), then it'll be marked by IOS as an active app and so won't be archived off to free up space. At least that's what Android seems to do.
I personally don't have a problem using apps for travel - far easier usually than fiddling around with paper tickets. For my last trip to France, I managed to use the train from Essex to Liverpool Street, underground to St Pancras, Eurostar to Lille, then unlock and drive away in a hire car, all using apps. We both had the apps on our phones, so had backups in case of any problems.
Did the same a couple of years ago, with train/DLR to London City, flight to Geneva, then travelled around Switzerland by bus, train and boat, again using just a few apps (one to cover all 3 transport methods there).
I've had far fewer problems with etickets than I have with paper ones. The only real issue is the lack of barcode readers at Stratford, so have to request to be let through the barriers. But that's less of an issue than the usual one of paper tickets failing to read properly at some stations, but working perfectly at others.