The adult content is what differentiated tumblr from other platforms, users came to tumblr for it and stayed to view other content. Remove it and those users decide it's not worth sticking around versus twitter/facebook/whatever. So while you don't lose as much content you do lose a proportionally larger percentage of users (50% seems to be cited). And once those users leave the creators of sfw content will start to move to other platforms.

Or in other words porn was free marketing and lead generation for tumblr. I somehow doubt a buyer will be willing to put in enough money to make up for it.