If it doesn't meet the reader's needs the reader can go elsewhere. And if it ends up being a better experience they'll gain viewers. That's kind of how things are supposed to work.
It's not clear that what Medium has broken anyone's configurations or stylesheets. I mean, if Google has dropped underlining links altogether, wouldn't that also break things? And since Readability has no problem imposing its own style on Medium's content, it's difficult to see how this could be all that difficult to overcome.
I'm sure there are lots of real-life challenges for blind people. I'm not sure Medium's underlining is a major one in comparison, or at all. They're not providing an essential service. They're a design-oriented website. Design choices are obviously important to them, and I don't see why they shouldn't have the right to design integrity. It's also not like they implemented this in a way that breaks accessibility when a simpler alternative existed. It seems a bit like blaming Lamborghini for designing cars that 300 pound people can't fit into, or that cant be easily adapted for those in wheelchairs.