They'll only eliminate it with respect to SCOTUS nominees. If they can avoid eliminating it for other legislation, they will (and they probably can). That would mean we'd have to broaden the elimination of the filibuster in order to revise the number of justices on the court.

Second, if we threaten that, why wouldn't the Repubs get rid of the filibuster and add a couple of more justices now, giving themselves an even larger majority -- one that could be 8-3 or greater by the time we were in a position to add justices.

Finally, whether FDR's threat to pack the court was "successful" is a matter of significant debate, with most scholars regarding it as a significant defeat for FDR.