What? We sold tickets for literally decades upon decades before smartphones came out. Of course you can do it, it's already been done!
Even if we could make them truly unforgeable, people generally want electronically transferrable tickets. How do you propose to do this?
Don't bother your season ticket holders about getting their own person admitted! I am standing in front of you, bearing identification, and you are whining about a mobile app?
The real reason, I fear, that we need the apps is data harvesting to be sold to data brokers.
They could even do something like give him a little RFID token that can be used once. Tap it, gates open, go in, done.