You're asking people to predict the future...
I can only assume that Yahoo was aware of the vast amount of Porn on Tumblr way before they thought about buying the site. And Marissa is pretty smart, so she can also come to the conclusion that if they remove the porn, a majority of the reason why they bought the site, pageviews, will go away.
Now, are there any legal copyright issues that can now be lodged at Yahoo? Possibly. So we'll see what happens. If they do setup to remove the Porn, it'll only open up the market for a competitor to take over. Interdasting...
I totally agree with you! there will most definitely be competitors to take over.