That would make it a hardfork, not a softfork, if done exactly as you say. 

Segwit only separates out signature data. The 1 MB limit remains, but would now only cover the contents of the transaction scripts. With segwit that means we have two (2) size limits, not one. This is important to remember. Even with segwit + MAST for large complex scripts, there's still going to be a very low limit to the total number of possible transactions per block. And not all transactions will get the same space savings.