Basically, I don't think there's going to be an announcement that Tumblr is getting shut down. If the pattern we're seeing holds, I don't even think it's going to happen intentionally. I think Tumblr is going to go out like an early 2000s forum: they're just going to keep cutting the tech support budget until something goes wrong in a way that none of the remaining skeleton crew know how to repair, which could be tomorrow or three years from now. If it does happen, it's probably going to happen completely at random, so there's no point stressing out!
The duality of tumblr going down