I don't even think Hashirama can 100% defeat the Juubi on his own anyways....it does have a size that is either as big or bigger than his new summon. Not to mention its increadibly OP TBB laser and its large scale Mokuton it was able to use on command. Worst part: It still has another transformation.
The only problem to this theory is 2 things:
1. Madara is no longer connected to the Juubi. Unlike the Juubi, the armor ws placed when Kurama was under his control, and as such it may be a requirement to wrap his Susanoo around the Juubi.
2. Going along with the Connection concept, with the connection lost, the Juubi may not allow a reconnection between itself and Madara. Before this time period it existed on its own, and did its own desires until Rikkudo Sennin defeated it. As such, it may not allow a second attempt at a re-established connection!