If a communication norm is just about other people’s preferences, why should they change? Who’s to say that other people’s preferences are more important than their own, particularly given that this particular preference is shared by millions of other people.

If inconsistent use of capitalization actually hinders understanding for some subset of their audience, then that’s a different story. My experience is that people are more likely to be annoyed than to actually have issues understanding all lowercase text. All caps text, on the other hand, is a different matter - and plenty of government and corporate entities are fine putting important text in all caps. But all caps text is a known accessibility issue. When I search for “all lowercase accessibility,” though, all I get is a bunch of results saying to not use all caps text for accessibility reasons.

If you have sources showing that all lowercase text is an accessibility concern, then you should share them. Heck, you should have led with that. But as it is, your argument ultimately boils down to “someone else should change what they do, that works for them, because it annoys me.”