I thought the grubs always exist and it is the case of infestation (exceeding some number/sq ft) that they become problem manifesting in dead grass patches on the lawn. They eat the grass roots close to the surface until they emerge to fly away. With this kind of situation, there is small chance of fatal or serious damage to the fig tree root system (fat roots, secondary roots and feeder roots). Even if the grubs nibble at some feeder roots, consider it as mini root pruning.
This is not an expert opinion, just some trust in fig determination to survive odds.