How do people not know how to get or use draw.io? You literally go to draw.io and can use it online.
We're replaced our diagramming with it instead of things like omnigraffle as it means everyone has access to tweak the diagrams without having to buy software (and then buy upgrades and plugins), and by embedding the diagram in the .png there's just one file to keep track of.
I've worked at places where people have exported draw.io as PNG/PDF on internal wikis for diagrams. But what happens when you want to change it? You then have to track down the people who made them, and get access yourself - or find it buried on someone's google drive.
With PlantUML you usually commit your diagram's text syntax to VCS, often close to the code.
As far as usability, we're never going to agree. I find it tedious, compared to writing predictable declarative syntax.
We're replaced our diagramming with it instead of things like omnigraffle as it means everyone has access to tweak the diagrams without having to buy software (and then buy upgrades and plugins), and by embedding the diagram in the .png there's just one file to keep track of.