When I was a student, I got an oyster photocard for commuting with a discount. Eventually I also had my railcard added to this (though IIRC, the discounts aren't cumulative). I had it renewed right at the last possible moment before expiry and aging out, and the new card was meant to expire on the 31st of Jan 2020. It never did and I've been using it since — maybe expiry meant the discount?
Eventually the outermost plastic layers peeled off (the layer with my name and photo on it) leaving an ominous blank card.
The card number was also peeled off, so when I had an incomplete trip one day, while getting that sorted, a friendly TFL employee let me know what it was on a receipt of my past few journeys. Only then did I really think about what the point of using an oyster card is (since I'm not getting discounts anymore) over a contactless credit card.
It seems there isn't really much of a benefit for me, so I'll probably just let it run out and stop using it. I might draw a little picture in that empty spot.
I had a normal oyster card many many years ago (before the first photocard) that I at some point added to the online dashboard with 60p still on it. I had given this oyster card to a homeless lady thinking there was more than that on it and she probably tossed it. I reckon if I plan my last trip in such a way that the balance goes to -60p, then never top it up again, then my overall balance with TFL should be... well, balanced!