The issue is that people cannot comprehend why someone who is well-informed and in sound mind would regard both potential outcomes as having absolutely no perceptible difference. I've seen it a few times, now, the process starts off with a reasonable statement of the distinction between the two platforms and what they mean for society going forwards, but - for whatever reason - that distinction isn't sufficient for the listener, and so it boils down to a perception - whether intended or otherwise - that the listener regards both such outcomes as functionally identical.
I think, when you get to the point that you can't comprehend why someone would have that opinion, you start to think that there's some layer of prejudice or wilful ignorance at play, and that's when frustration comes in and the more aggressive rhetoric starts up. But I would also argue that, well, if there is prejudice or wilful ignorance at play, the person being addressed is already lost; it's incredibly hard to argue against an inaccurate worldview if someone has absolute unshakable faith in it, even if it's incorrect, and the internet isn't the best place for a calm and nuanced takedown of those arguments.
That's largely referring to the general case, but I'd also add that in this specific instance, it's very easy to perceive such a stonewaller as having a fundamental lack of compassion for communities that would be adversely affected by a Trump nomination, and that will make people angry.