Shiryu is implied to be his RHM and/or the second strongest member
So either way he'll be destined for great things. No reason for Teach to surpass Xebec while Shiryu is below Xebec's strongest subordinate (Primebeard)
The problem with Shiryuu: He is not someone to be the strongest, he is not someone who magically became a high top tier.
We not speak about a regular top tier, we speak about Whitebeard, who stands above other top tiers like Kaido and co.
And let be honest here, you think current Shiryuu could match guys like Kaido/Roger?
Teach/Xebec make sense since Teach is Luffy opponent, but the others doesn´t fit in.
Whitebeard was peaked with everything, insane strength, insane haki and insane df.
And now look at Shiryuu, who almost got knocked out by a basic throw of Old Garp.
Shiryuu df is not even close to Whitebeard df.
I can´t imagine Shiryuu to be strong as Whitebeard, not even with a second df.
Same counts for the other Blackbeard pirates, no one is gonna replace monsters like Shiki,Linlin or Kaido...
They gonna lose to guys like Nami,Usopp and Jinbe in the end...
And the reason is clear why, Whitebeard and the others didn´t had counterparts in Roger crew, they directly RIVALED Roger himself. There is a reason why Whitebeard,Linlin and Kaido fought Roger and Garp and not anyone else, they been legends on their own, while Sanji and Zoro counterparts will never rival Luffy as example.
We can´t compare Rocks crew with Teach crew.^^