I can only speculate, but from what I’ve read elsewhere, Tumblr would’ve run out of funding money soon, so at some point, the cat-lovers and cutters would have had to grin and bear it or take their business elsewhere either way.
Rumor has it that running a service (especially one used by millions) costs money, and since I first joined Tumblr a few years ago, I’ve paid precisely fuck all, which is what I assume everyone else paid too.
Fear of change is natural, and sure, Flickr has definitely been mismanaged until fairly recently, but still – take heart, kink lovers: there’s loads of skeevy porn still to be found there, Yahoo! takeover or not.
Furthermore, provided these (so far, entirely) unsubstantiated rumors are true, Yahoo! has agreed to mostly leave Karp to his own devices for the next 3 or 4 years, at which point there’s a good chance Google will own and run everything everywhere anyway.