> Are you really arguing that all operators introduced a yearly fee of exactly the same amount, and in a very short period of time, was somehow perfectly balanced out by a drop in monthly rates, and on average nobody ended up paying more?

I'm arguing that after 2011 when the first operator introduced those fees, the cost of mobile phone plans decreased for many years until around 2019. It's also that not all operators introduced those fees and not all operators did at the same time. The main reason why many operators started charging with this fee structure is pretty obvious: it makes the monthly cost appear lower than it is. It's a form of sanctioned price in-transparency.

//EDIT: the main reason for the introduction of that Servicepauschale was that for a while internet service providers where required to participate in the now illegal "Vorratesdatenspeicherung". The pauschale was generally understood to cover the cost of the infrastructure for it.