As someone who has looked into building something like this, a subscription makes a lot of sense. These GTFS feeds are (generally) a huge mess. Cleaning them up into a dataformat you can use is tons of work. And with all the cities changing their routes (or cutting service) every six months, it means continued work and support.

I'd happily pay a subscription for something that works well. But yes, it will be interesting to see how naive consumers react to this.