Question

If Volantis was destroyed would that reroute traffic to slaver’s bay?

Probably not. Keep in mind, the economies of Essos are fairly specialized. In this case, the cities of Slaver’s Bay act as processors of the human raw materials that the Dothraki extract, and sell them to the Free Cities:

“For centuries Meereen and her sister cities Yunkai and Astapor had been the linchpins of the slave trade, the place where Dothraki khals and the corsairs of the Basilisk Isles sold their captives and the rest of the world came to buy.” (ADWD, Dany III)

 Of the Free Cities, Volantis was their largest customer. Thus, Dany’s anti-slavery crusade has had two effects. First it’s terrified the slaveowners of Volantis into electing a tiger majority and going to war against free Meereen:

“The best calumnies are spiced with truth,” suggested Qavo, “but the girl’s true sin cannot be denied. This arrogant child has taken it upon herself to smash the slave trade, but that traffic was never confined to Slaver’s Bay. It was part of the sea of trade that spanned the world, and the dragon queen has clouded the water. Behind the Black Wall, lords of ancient blood sleep poorly, listening as their kitchen slaves sharpen their long knives. Slaves grow our food, clean our streets, teach our young. They guard our walls, row our galleys, fight our battles. And now when they look east, they see this young queen shining from afar, this breaker of chains. The Old Blood cannot suffer that. Poor men hate her too. Even the vilest beggar stands higher than a slave. This dragon queen would rob him of that consolation.” (ADWD, Tyrion VI)

The other effect that it’s had is to raise the price of slaves by massively cutting the supply. Hence why in ADWD, you see slavers being so greedy and reckless as to kidnap wildlings and try to sail through Braavosi waters, because the pure profit is worth the risk of being hanged. 

Thus, if Dany attacks and captures Volantis, the economic effect will be to massively curtail the demand for slaves, by removing one of the biggest markets for slaves in all of Essos – because Lys, Myr, and Tyrosh aren’t big enough to pick up the excess. Which may be enough to break the slave system throughout Essos…