So what happens when a platform grows and that threshold is reached one day? Force everyone to de-anonymize and potentially reveal sensitive information about themselves or abandon their account?

There's just no good way to force only some to de-anonymize without running into problems.

While I believe in the right to online anonymity, I also don't think that de-anonymization would even work, when I see the same garbage being posted in places that enforce real names. It just doesn't seem like a detractor to those types of people.

Instead, I'd rather want to see harsher punishments for big sites failing to moderate their content. I'd also take a look at these personalized "recommendation" engine and maybe ban them altogether. (Bonus points if it also affects personalized ads.)