Well, no. Only for non-fully-validating nodes. All nodes that ignore the signature chain will need to trust other -- fully-validating nodes (which get no capacity boost) -- to do the validation for them.
It does for those who upgrade; those that don't upgrade never needed the increase in capacity.
I can't tell what is subject and what is object in your reply. But if I have your words parsed properly, than I believe you are making a false statement. Let me try again.
For any given node:
- In order to operate in a trustless manner, a node must fully validate
- In order to fully validate, a node must have access to signature data
- SegWit partitions transaction data into two chains - the operational data and the signature data
- the 'capacity increase' that SegWit claims is entirely due to the fact that they don't tabulate the signature data as being part of the block size accounting
Ergo, any node wishing to operate in a trustless manner does not get any 'block size increase'
You may reply 'but fraud proofs'. But this is yet another mechanism entirely dependent upon outsourcing validation to other nodes. IOW, not trustless. Insecure.