I am not on discord, so I don't have skin in the game. But it depends on the community I guess. If everyone stayed on discord then there would be no change. But any kind of change would probably have some kind of effect, even if all people migrated to a new platform.

I can try to think of a simple scenario. For whatever reason, a user may not be as active in the new platform as they were on discord. This could alter the community dynamics. On a bigger scale this could have visible effects in the actual community.

To be clear, I am not advocating in favor of staying on discord. I just find the concept of community building interesting.

This is a completely different topic though. And not relevant to point being discussed. Analyzing the effectiveness of community building by different groups is a separate issue.

I don't think the article proposes that a community should not accept new members. On the contrary, it critiques the breaking of communities.

Migrants or refugees have to find a new community because their old one was broken for whatever reason, be it war, financial troubles or something else. So in that case, that first breakage of community should have been prevented and the community preserved.

The author of the article claims that a mere migration to a new platform does not solve the problem. It just fragments the community. I agree with that. For one or another reason not all people will migrate.