You make a fair point, but if the replies to a Lemmy post are overtly hostile, I see that as a failure of moderation rather than of the system itself. Twitter has long been an “almost anything goes” sort of place and was never purposely designed for community-oriented discussions. It’s a microblogging site with discussion features grafted onto it — way back in the day, the Tweet box literally asked “What are you doing?” and there was no reply function so people posted their own “top-level” (there was no other kind) Tweets along the lines of .@user_xyz “Lorem ipsum dolar” My opinion is blah blah blah but that wasn’t automatically shown next to user_xyz’s Tweet for all to see.

Also, I’m not certain of this, but I think displaying downvotes without enabling them would require a software patch. I also suspect it would be confusing to many users and lead to a lot of bug reports.