Can't is too strong of a word. Enabling it would be confusing though simply because we're used to credit cards creating a charge rather than acting as identification.
Step 1: Allow registration of multiple credit cards, debit cards, etc. to an account. This is strictly for identification purposes; the number simply says "I am this person" and is not for charging purposes.
Step 2: An web/app interface which allows moving the pass between registered cards.
So, a pass can only be registered to a single card at any given time but it can float. So you carry your creditcard and your 13 year old carries their bank debit card for example, with the pass shifted from one card to the other depending on direction given to the backend.
The obvious downside of making it that easy is pass sharing would become very common. You could get several rush-hour trips out of a single virtual Presto card. I expect phone-based Presto implementation will be device specific, including independent of any physical Presto cards for a similar reason.
Metrolinx Wouldn't rather than Can't is a better term.