The point isn't that voicemail is super important; the point is that infrastructure wasn't even secure a decade and a half ago. The systems you're talking about were all exposed then too.
Everything might be more secure, as you say, but there are so many more ways a small hole could be exploited to do damage now.
[0] https://www.popsci.com/scitech/article/2009-08/first-patient...
One only has to look at self-driving cars to disprove you.
Dan Geer also entirely disagrees with what you wrote [1] [2] and you're no Dan Geer, sorry to say..
† Sure, I just did, but I'm being upfront that it's a cheap and unfair thing to say. I'm human.