I just don't see it happening here, at least within 20 years. I don't know if you've seen the swaths of people complaining online to Presto about privacy concerns and how they only want to load the card with cash and how it's completely unacceptable that the TTC's only cash location for Presto loads is Davisville. I personally only ever use my credit card via Apple Pay for Presto loads at SSRMs, and for just about all purchases in general, but many people--mostly older folks, but even some younger people--simply refuse to use cards, or at least avoid it like the plague.
At subway stations there's not much point in eliminating cash since there's space for machines that accept cash. They are planning to have vending machines that dispense paper-based one-time-use disposable "presto" cards for single rides, which you will be able to buy with cash, debit, or credit; you'll be able get a disposable card or an actual card (or load your existing card) at those. They'll replace the Token and Pass vending machines and probably the current Presto reload machines which were never intended to exist and just popped up as a stopgap before these big machines were ready to deploy.
On LFLRVs and buses, one thing to keep in mind is debit/credit payments may not be possible because they sometimes lose cell reception. For instance, the 509/510 at Queen's Quay Station usually shut off their credit/debit option as soon as they get underground due to signal loss. So then people wouldn't even be able to just hop on a vehicle, they'd have to walk to a subway station elsewhere to buy a card then go board their vehicle. This is not an acceptable way to run a transit system for either locals or tourists.
Aside from the reception issue, on LFLRVs there's not much point since their fare payment machines already take cash. There's no reason to fix something that isn't broken.
As for buses, consider dropping cash on all of the TTC's massive bus network, much of which is in areas that there is not, and likely to ever be, convenient Presto purchase opportunities nearby. People might have to take a $20 or $30 taxi ride to get somewhere that they can buy a card. This will almost completely eliminate casual and visitor ridership. For the bus network at least, I think keeping the farebox makes sense. And would there really be a benefit in eliminating it?
At any rate, I've never heard anyone at the TTC even hypothesize about the possible merits of eliminating cash within the next 20 or 30 years, and all discussions I recall on the Presto topic have emphasized that cash will always be an option.
