I had a 15 year old ereader from sony that could open pdf, epub, rtf, txt whatever you wanted and text reflow would work for everything bar pdfs.

Kindles are actively walled to force customers into the amazon ecosystem.

I could just plug this sony e-reader to my PC and copy whatever I wanted to read. The e-ink panels have gotten substantially better but the experience is now objectively worse with Amazon and more expensive, most customers not don't realize what it could be.