I disagree with this sentiment. It is entirely possible that there will be people who are regulars on one platform who are just unable (actually unable or perceives themselves unable) to migrate and the morale lost from losing their regulars is huge. Or a subset who insist on staying, forming their own sub-community, and neither the migrating group nor the people who insist on staying produce enough engagement for the members and so the community as a whole fizzles out. This is all squishiness. There is a reason why deplatforming appears to work in reducing the effectiveness of political groups, even if the people who remain in the community post-deplatforming are hardened in their loyalty to the political policy of the group.