I think Oyster really is tourist-friendly. When I'm leaving the
Netherlands I'm not going to get the value left on my strippenkaart
back, but a tourist in London can just hand in his Oyster and get his
£3 deposit + any value left on it return. Next to that, Oyster cards
never expire which the Dutch strippenkaart does.
I hope the complete switch isn't far off. What happens if a tourist
goes up to the ticket window and asks what might be the best option for
him if he's staying here for a week, for example? Are staff instructed
to tell them about the advantages of Oyster?

Some even more clear signs aimed at tourists at the stations that they
usually enter London through (All the terminal stations and Heathrow /
LCY basically) might make it a bit more clear.