So, to argue against using a platform supported by Nazis is "high minded moralism"?

Yes. Not much to add here, it's a moral argument not a practical one. And trying to make it a practical argument will yield very poor results.

Convenience and mass usage is an excuse for that support?

Usage does not equate to support, most people aren't even paid users. Besides that, it's not just a matter of convenience, it's often a necessity. Try being a Latam journalist without an Instagram, a tech workers wiwithout linkedin, a communicator without YouTube. Absenteeism's only effect is giving you a warm fuzzy feeling of moral superiority. For serious militants, if you want to have an effect on society you will unfortunately have to engage with it.

Besides, this discussion started from arguing that a guy being beaten up by the police due to being doxxed by either Zionists or Banderites is somehow justified because he... Uses twitter? If you can't see the frankly absurd implications of that, imagine if the same happened to Richard Wolff, Vijay Prashad or any other communist who has an account there. Or any other random person with an audience, in fact. Imagine if Jenny Nicholson, NileRed or any other non-left non-politics influencer randomly decided to oppose Zionism and Banderites online and got physically attacked, since you agree YouTube is just as bad. Not even getting into MS Windows users. Are they all Nazi collaborators?

I truly hope you simply haven't thought this through. Sorry to tell you, but hiding away in a commie forum with 200 users and belittling anybody out of it is not praxis.