The total cost to the tax payer is around 3 Billion euros, which as a subsidy is pretty cheap in comparison to the 3-5 Billion euros of subsidies for company cars.
I'm not sure on the current data but during the 9 euro ticket trial over last summer 10% of daily commuters shifted from cars to public transport, 17% overall shifted to public transport and 1.8 million tons of CO2 were saved. The company car subsidy on the other hand is definitely NOT climate friendly.
I'd like to read arguments beyond "great, tickets are cheaper!".