I just feel like if all of the left wing voices abandon twitter, the right-wing and fascist voices will become more common in general as twitter is where most people are. You cannot just let them exist and thrive unchallenged until it's just not possible I guess. This will alienate people becoming more leftist if the average person, who just browses twitter because it's popular, constantly sees more and more unchallenged bad takes.
Truth social was born as a right wing platform from the beginning. It never had and will never have any casual users who are not right-wing. Going there and adding a left voice will not be very helpful as you would talk to an already established hard opinionated community. Twitter is unlike that, there are a lot of people who do not care about politics currently or are more casual about it. Though if you keep seeing more and more right-wing stuff, such people get normalized to that. Instead if there's a left-wing voice there, any non-politicized person will have a look at the other side as well w. This is how you win over more people to the left, by being active on places where the most people will hear you. If you leave this place you just surrender and have more casual twitter users lean to the right as the time passes.