Default behaviour when network partitions/pruned network
Bitcoin Core does not support pruning witnesses. It supports pruning blocks entirely, but that is independent from segwit.

No full node in the network will accept a segwit block without the witnesses. It's simply impossible to validate a block without it, so not having a witness is treated as equivalent to not receiving a block at all.

SegWit does enable dropping witnesses for relay. It is a bandwidth optimization for lightweight clients, because they don't care about the witness data in the first place.

In theory, nodes could indeed choose to not store witnesses at all, but why bother? They can already delete the blocks entirely if they don't care about serving other nodes that want to verify history.

All pruning node have a minimum requires number of kept blocks, which corresponds to a few days of time. That should be sufficient to overcome reorganizations and temporary partitionings.