[nerd_light]

I think that your consideration is a good one, but I think it comes down to numbers. Will a no-additional-cost-per-ride pass increase the "unnecessary" load on the system so much that they need to start using additional transit vehicles, thus increasing emissions? What percentage of trips are "unnecessary"? Will the growth of the system for "necessary" trips be able to handle the "unnecessary" load anyway?

Aside: for "unnecessary" trips, I'm setting aside the possibility of other options, such as "I already own a car and will now use that instead of paying 1 EUR".