If you make it easy enough and obvious enough to figure out how to buy a card and don't impose a significant card purchase fee, I think a lot of temporary visitors are willing to do it. The problem is when transit agencies don't make the process of obtaining a card as straightforward as buying a single-trip ticket.

Personally, I've got around a dozen transit cards spanning at least six countries...

The Oyster card in London is the only way to pay. Pay stations can vend cards, but they need to be distributed more for this to work with Clipper.