i totally agree with you, im just a user of a theme, but they asked us to use OAuth in case if we cant install their demos, here is the link of the theme im talking about:
https://bringthepixel.ticksy.com/article/11788/
here is what is mentioned on the link:
15.After successful activation I still can’t install the demos.
Please try using Global OAuth Personal Token instead of Single Use OAuth Personal Tokens in Envato Settings.