At this point, what would showing his taxes even do? Show that he doesn't pay them? His constituents don't care. They'll just claim its more fake news. Hammering on his taxes will be as pointless as hammering Hillary on Bengazi.
It. Is. Not. About. His. Most. Devoted. Followers. Anymore.
Are people going to do this every time Trump is investigated or calls for transparency are made? People care. Did you see millions of people marching in the streets yesterday? People care. Did you see people protesting legislation rolling back the ACA several days ago? People care. There are a lot of issues related to this presidency a great many people care about.
This site is attracting attention. People care. A lot of people care. We're not going to sit on our hands over the next for years because his most hardcore followers "don't care" about this or that.
EDIT: Also, ultimately, your example about hammering Hillary on Benghazi is flawed. Although years of investigations ultimately turned up nothing substantial against her, the years of investigations themselves had a cumulative effect of creating a cloud of distrust against her. Between Benghazi and emails it created a narrative among many voters that she was "always under investigation". That turned into a serious issue that dragged her throughout her presidential campaign.