Transfers will be fine - and Presto isn't going away. Debit/credit/mobile wallet will only offer a single adult TTC fare at this time.
Therefore, until there's weekly/monthly fare capping, debit/credit/mobile wallet is useful for people who use the TTC on an irregular basis or a few times a week. It's also great for tourists and occasional riders who don't have to hunt down a Presto card or find it in some kitchen drawer or whatever. It's also useful for people who've left their Presto card at home! If anything, I see the debit/credit/mobile wallet replacing 90% of use cases for the Presto ticket.
Riders that want monthly/yearly passes, or student/senior riders who want discount fares - will use Presto. It's also 5 cents cheaper per ride - which adds up over a year! There's also plenty of people who don't have (or don't want to use) debit/credit cards to pay.
For low income riders - a Presto card will run out of money when you take your trips, whereas using debit/credit/mobile wallet you get charged the total fare amount at the end of the day. For some, this can be harder to keep track how much has been spent!