The problem is that there seems to be a serious amount of fraud there. I suspect the susceptibility to fraud is at least partially self-inflicted here, but the Deutschlandticket is well known to have been abused in various ways.
So the providers want a live verification, not a static QR code that always remains valid for a month or so. My understanding that this is both, to combat fraud where fake tickets are sold (well, real ones but bought with fake or stolen payment methods) as well as people using tickets other than their own. The latter is theoretically prevented by checking photo ID, but almost nobody does that in practice.
So they force the app because it's slightly harder to fake than just copying the QR code. I don't think you'll get a more open system here unless the underlying vulnerabilities are fixed in some way. And you'd probably need to embed photo ID information in the ticket somehow to prevent multiple people from using a ticket, and I doubt that will happen for privacy reasons.
So the providers want a live verification, not a static QR code that always remains valid for a month or so. My understanding that this is both, to combat fraud where fake tickets are sold (well, real ones but bought with fake or stolen payment methods) as well as people using tickets other than their own. The latter is theoretically prevented by checking photo ID, but almost nobody does that in practice.
So they force the app because it's slightly harder to fake than just copying the QR code. I don't think you'll get a more open system here unless the underlying vulnerabilities are fixed in some way. And you'd probably need to embed photo ID information in the ticket somehow to prevent multiple people from using a ticket, and I doubt that will happen for privacy reasons.