Some of us are suffering from a sort of whiplash... we've been told (by some factions and their hangers-on) for months that raising max block size even to 2MB is highly dangerous for decentralization. But now, completely reorganizing some of the basic functions of the protocol, with a (somewhat unnecessary) requirement that there be no hard fork... has led us to the point where the same group with those concerns... is offering a fairly drastic solution that effectively raises the requirements for fully validating nodes to a 4MB(or 2?) max equivalent.
It's weirder than that. The 'drastic change' (i.e. moving the signatures to a separate data structure) does absolutely nothing to address scalability for fully validating nodes. To fully validate, such nodes need all the block data and all the signature data. No reduction there. I merely reduces demands on _non-validating_ nodes, by a factor of 1.8x or so.
What the entire SegWit proposal does to address scalability at fully validating nodes is not the segregation, but rather a simple _increase_in_the_block_size_. In Wuille-speak, this is represented as "Discount witness data by 75% for block size Or: block limit to 4MB, but only for witness".
At least as far as I can tell.
Bait & switch?
standard disclaimer: I have an incomplete view of SegWit at this time.