It is great that you ask a question, because we live in a world with the freedom to opine on things. What could be considered a massive issue to me may not be a massive issue to another; and if we feel the world will be better by debating our positions, we have the right to do so.
Today, anonymity and pseudonymity exist and allow people to speak freely without risk of backlash for having a different opinion as often times the right opinion may differ with that of social consensus.
If KYC is introduced, the ability to maintain freedom of speech, online, will likely diminish.
This is of negative consequence to the people of the world.
Further, with internet 'forever data', LLM NLP and so forth, character profiles are too easy to develop for people which can cause further harm as we begin segregating based on said profiles.
I believe this KYC requirement can even extend to blockchain node operators and so forth as well.
These are just a few reasons but there are many more.