Indeed, that'd be a more useful metric. Very hard to measure well though and probably actually leaves open more room for them to game the metrics than the current system does.
Keep in mind that for the majority of trains in Germany, nobody bought a specific ticket for that journey. We just use the DeutschlandTicket which is a flat subscription of 58€/month, which gives unlimited access to busses, trams, and regional trains (basically everything but high speed trains).
With the deutschalnd ticket, you typically just walk onto a train of your choice and go wherever you want. They dont actually know where all the travellers are going or even how many people there are
Yeah I know you cannot track all journeys. Deutschlandticket, BahnCard 100, and similar things will be invisible. You can track the ones booked through the app though, and with a high enough sample rate you should get sufficiently close to the truth, unless there is a bias I haven't thought of yet.
You could also select random virtual journeys, I suppose. In arbitrary units you should at least be able to measure whether DB is improving or not. You could even delegate this to an independent organization. Actually, now that I think of it, isn't the API public? I'm reminded of the talk by D. Kriesel about DB data mining.
Keep in mind that for the majority of trains in Germany, nobody bought a specific ticket for that journey. We just use the DeutschlandTicket which is a flat subscription of 58€/month, which gives unlimited access to busses, trams, and regional trains (basically everything but high speed trains).
With the deutschalnd ticket, you typically just walk onto a train of your choice and go wherever you want. They dont actually know where all the travellers are going or even how many people there are