In the short run, yes, they'll retain market share.

But, as always with the left, overreach will happen. They will get emboldened, complacent, and more comfortable in their dominance than they already are. This guy doesn't seem to really care how things look to the outside world, or is more focused on marketing to the Beautiful People (only the leftist jerks), so I think there's a good chance long run more people will get driven away, and those already radical will radicalize further and continue to convince themselves they are the true majority in the country. Because, after all, 'everyone' on twitter is like them!