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.