It’s not just about how apps handle it. Sometimes you just want to show or hide it. For example you want to read something on text-editor, so you hide OSK. Than you decide to write something and you enable it. Or you want to use a keyboard shortcut and open OSK to hide it after that shortcut. Of course it depends a lot of users and how they interact with apps etc. But that’s why I didn’t say “replace long press”, but “add an alternative as second way to interact” in post 3.
I absolutely agree with you in case I would be someone who just types something when there is a textbox which auto-open OSK and auto-close afterwards. But that’s just one use case of many.