A fan written ending is somehow "disrespectful to Isayama". Or thinking a fan ending is better than the actual ending is disrespectful?
Harrassment and death treats are disrespectful. Criticising and saying you didn't like a story isn't disrespectful. And these dedicated fans who chose to write a fan ending are the ones facing harrasement and being disrespected. Isayama is an adult who chose to join one of the most pressurised writing industries, criticism is to be expected.
Even students who write anything in university get they shouldn't write things they don't understand. Or they research until they do understand. This applies to many industries like music, literature, science and med