Thanks for your reply and help, Xynerise.
My question was because I read this:
So we agree that, on average, the block size will increase approximately 2x in Segwit. With that 2x size increase we will also get 2x throughput? If so, we could have done just as well simply increasing block size without Segwit.
My question was because I read this:
Quote from: Jimmy_Song
When serialized to a Legacy node, they will be 400 bytes, 300 bytes and 500 bytes respectively (tx size without witness data). When serialized to a Segwit node, they will be 800 bytes, 1100 bytes and 500 bytes respectively.
If the witness data isn't included in segwit transactions then how do you validate a transaction without the signature? 
The same way you validate that same transaction when it's included in legacy blocks? Or do legacy blocks need not validate segwit transactions? If not, why not? As soon as we discovered we don't need witness data in legacy blocks why not simply cut them out?So we agree that, on average, the block size will increase approximately 2x in Segwit. With that 2x size increase we will also get 2x throughput? If so, we could have done just as well simply increasing block size without Segwit.