If Blackbeard brought Shiryu along to Amazon Lily, he probably would have acquired Hancock's DF successfully. Shiryu could take care of old Rayleigh while Blackbeard took care of Hancock.
Come to think of it, most of Blackbeard's career was carried by Shiryu.
He wouldn't survive against Magellan, he wouldn't recruit Impel Down prisoners, he wouldn't get Whitebeard's Gura Gura, he wouldn't become a Yonko like he is now, if Shiryu didn't save him.