I agree not much can be done, but why drop it? Getting some free press to the tune of "get great features 18 months earlier by switching to HelloSign" can't hurt and writing a blog post only takes a few hours.
I'm lost as to the mentality of, "Just drop it." Why just drop it? Recently, I had to bring legal action against an ex-employer for breaking the law. His advice after he fired me was also, "Just drop it, because the valley is a small place." D@amn straight it's a small place, so if you're going to out and out clone a competitor, you should also expect to see the mob of villagers arrive with pitchforks and torches. Seriously.