Neither of those comments address the CX problems. What is a fare inspector supposed to do when someone pulls out a bricked phone and says "sorry, my phone battery died"? What is a CSR supposed to do when someone calls them saying they couldn't tap off because their battery died?
In the case of the fare inspector, fine the passenger for being unable to produce valid proof of payment. If you're using your phone as your farecard, you should make sure that it'll be operational for the duration of your trip. For the CSR, tell them that's unfortunate, they'll have to pay the missed tap off penalty or miss any transfer credits. You can also put phone chargers at subway exit points.
It should go without saying that mobile payment would have to be strictly optional. Presto would primarily remain in the form of a dedicated card--migrating it to your phone would be an option for those so inclined. At some point, adults need to be responsible for their actions and
A much more practicable solution, which is how the TTC will eventually work (their Presto card readers are designed with support for it) is contactless credit card payments - if someone really wants to use mobile payments, they just pay directly from their credit card (which can be on their phone)
Except that the cash/credit card fare on the TTC is $3.25 and the Presto fare is $3.00, and that multiple is likely to remain similar or increase (YRT/Viva is $3.50 Presto and $4.50 cash/credit/debit). The TTC has always lumped Credit in with cash (see LFLRV SRVMs currently, and the previous Dundas/College stations' pilot project for credit card payments on Presto readers at turnstiles) not presto.
Also, you can't do metropasses on credit cards, nor can you set a credit card up as a child credit card to open faregates for free--for that matter, obviously kids don't have credit or often debit cards.
And say goodbye to inter-agency transfers/co-fares/fare integration, unless GO, UPX, YRT/Viva, MiWay, Brampton Transit, and Durham Region Transit all start taking credit/debit and work out credit/debit fare integration too--unlikely to be possible due to the need to keep credit card data secure and private.
Your logic of "if someone really wants to use mobile payments, they just pay directly from their credit card" can just as well be changed to "if someone really wants to pay, they just pay using Presto" and stop taking cash, debit, and credit entirely.