Post author here: I appreciate all the comments. Even the negative comments because it's important to know all reasons for opposition. I'll point you to another HN story today, ZenPayroll raises $20M [0].
They're not on a path to acqui-hire, they're not trying to pivot away from something you rely on, they have fantastic support (but more importantly a product that's so clear and easy to use you don't really need support).
We (SimpleLegal) are ZenPayroll customers and love it. They are the exact reason your company should seek out startups as vendors.