I'm a millennial and I work in IT, and I would choose a physical railcard over installing an app. An app is likely to use disk space and battery and could malfunction. If the app used GPS it would drain the battery even more. Also, corporations nowadays are using technology against us more and more often, with apps using data to download adverts, or invading privacy such as sending data such as GPS co-ordinates, address book or whatnot back to the mothership
Disk space? What kind of phone are you using, a Dom Joly-esk handset.......
One way that would persuade me to use digital railcards however, is if the railcard was issued using the standard ticket/wallet feature of the relevant Smartphone platform (such as in Apple Wallet), rather than an app that demands allsorts of unnecessary permissions else it refuses to install.
Looking at the 4 main travel apps I have on my (Android) smartphone, memory sizes and permissions are as follows:
MyMCard (WY Metro) - 44.91MB - Permissions: Storage / Telephone
National Rail - 24.67MB - Permissions: Calendar / Location
Northern - 25.45MB - Permissions: None
TPEXpress - 96.56MB(!) - Permissions: Location
Now quite why the MCard wants access to the telephone functionality I'll never know, but it doesn't ever use it so I may ask this of the devs. Otherwise needing location is OK for me, I allow Google to tell me where I am (sometimes incorrectly I have to say, but I never get fooled!), calendar functions are fine as it can be useful to drop in dates / times of journeys especially when I have a fair few booked in for meetings, social events etc, and storage is fair enough if it is going to securely store ticket data (providing of course there is a sever back-up!). The TPE seems horribly bloated memory size for what it is, although to be fair it does include functionality for the WiFi entertainment system that is being rolled out so it could be that.
But I do get what you mean, apps shouldn't need all sorts of strange permissions when all they really need is data, NFC, storage and perhaps calendar when allowed. (Oh and I am a 40-something, ZX Spectrum generation!!!!)