I truly think...
...that Tumblr is trying to kill itself, because no normal website would do such a stunt.
As for NSFW art in general there is the question: When does it starts to become NSFW? People have different tastes and kinks. You can still create NSFW art with fully dressed people. If Tumblr had issues they could had applied rules like most pages have(i.e. no penetration/sexual acts, no aroused genitals, no gore, etc), or split it into two versions (one exclusively for NSFW art). And hired mods to take care of it. But instead they were lazy and greedy and Tumblr quickly became too big. Now they realized in what kind of trouble they are and are now going full force towards the end.