All of those had weaknesses
1. The Borg were defeated,
2. The Replicators where held indefinitely in a statis field.
Now the Q - beings that were only (as I recall) weak against their own kinds - now THEY could be definite contenders.
The Borg were never defeated. Captain Janeway just returned to earth.
The replicators all returned to one planet. Just don't do that.
