Lock the rows before removing the last owner

BEGIN;
SELECT id FROM members WHERE workspace_id = $1 AND role = 'owner' FOR UPDATE;  -- lock the set you count
SELECT count(*) FROM members WHERE workspace_id = $1 AND role = 'owner';      -- now stable
-- if count <= 1 and the target is an owner: ROLLBACK with an error
DELETE FROM members WHERE workspace_id = $1 AND user_id = $2;
COMMIT;