it's not really a question about it being hard, it's a question of cost.
The biggest draw of tumblr was that it was a free area to express adult interests. If it required payment to use, it never would have been as popular as it was.
It takes mass quantities of money to run any kind of site that does any kind of image hosting. We even get a taste of it with websites like this one or Ponychan, which don't even save stuff forever.
It's no joke: Tumblr has not turned a profit for either Yahoo nor Verizon. Verizon's buyout of Yahoo was even inextricably linked to Tumblr's lack of a profit for Yahoo (along with other questionable business choices Yahoo made, for sure).
It is a money sink. The fact that it has stayed up for as long as it has is amazing, and probably only because they make money off different sources of revenue so the site isn't COMPLETELY down the hole.
If people truly are serious about leaving because of this adult content ban, it may be the death knell for the service. Or maybe it won't, maybe the removal of all pornographic content will reduce the load enough that it makes it profitable, or at least less not profitable, and it keeps going.
Either way, if things continue going this way and people don't change their minds or truly do have the resolve to stop using a site that has been part of their daily routine for years, Tumblr will fade from popularity.
So, no, it's not hard to make a site that allows it, but it is hard to pay for it.