SegWit is also intended up to open up some new middle ground security options-- e.g. making it realistically possible for nodes that verify a random fraction of the data; with a security model in between light an full.
It also addresses some of problems with validation costs that are quadratic in transaction size.
It's also, by far, not the only tool in the Increased-capacity-without-harming-decentralization tool belt, for example I recently described a new approach for signature aggregation which, if applied to the current transaction load would decrease transaction sizes around 30% while making verification somewhat faster.
