Bruh, the whole prophecy doesn't make many sense. Screw prophecy
Without Shiryu around, Teach was already game over by Magellan's hands. Low diffed. Sure, fate played along with this scenario.
The whole "only Nika can beat him" doesn't make sense. Teach is not an immortal being who can't die even if he got poisoned, sunken to sea etc. There are plenty of ways to kill him actually.
It's not that only Nika can beat him
It's about two elements - Fate and threats.
Luffy and BB is driven by the fate.
Rest are playing their role as well - knowingly or unknowingly.
Shanks is there to play his part as well - he's letting Luffy have his journey but at the same time he's acting as a gatekeeper.
Not to beat BB but to stop him from meddling in Luffy's journey before it's time.... before Luffy is ready.
This is one of the reasons why Shanks vs BB is bound to happen...
There are two kinds of school of thoughts...
1) Your actions decide your fate
2) Your fate decides your actions
One piece like many shonen tends to take the second route.... it's convenient because it lets author justify lots of things