Imo viability should be considered if the price that the government pays is worth it, the 2022 budget seems to be around €457.6 billion. 2.5 billion seems to be totally worth it, as you agree.

If the measure would be kept year-round, it would cost around 10 billion, which seems a tad high. Are these costs offset somehow by the symbolic price of the ticket? Until 15 of July they sold around 30 million tickets so overall over the 3 months, maybe 40-45 million tickets would be sold. That makes 360-405 million, so around 14-16% of the cost. I'm not sure if this money goes to the transport companies or is used to offset the government's costs. If the ticket stays, but the price increases 2x or 3x (that would still be an enormous bargain IMO), costs would be offset even more, probably.

It's debatable of the economical effect that this measure already had, taking in consideration higher efficiency of public transport, more disposable money for the population, freer roads that might lead to more efficient transport services etc. Except the flat cost for the government and the long-term cost of infrastructure (which might be huge) I really can't see many negatives for this measure.