I think as how the Presto system will be when it is first implemented, there is already very little incentive for tourists to use it - the entire TTC is basically out of reach and I daresay it is the TTC (subway, streetcars, perhaps buses), especially the downtown routes, that tourists would use the most rather than the suburban buses or the largely rush-hour-inbound-only GO trains, except perhaps the GO Lakeshore lines (which are expensive enough to exhaust the credit in 3-5 trips, and if a tourist is only going to use it once or twice then there is little reason for them to get a stored-value card specifically for that anyway). If/when TTC finally comes on board, the setup might have been modified already.
As for occasional local users, as I said the $25 is not a reasonable deterrent anyway.