@Realpra thanks for the answers.
-snip segwit-
Did I miss something? Seems like a very expensive way to solve malleability and not much else.
You've missed 85% of the benefits of the design. The fact the it increases effective space is largely an incidental (but useful) side-effect. That it also make it possible to verify blocks fractionally (at your random choice) without having to trust data from third parties is what I was mostly referring to...
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Id also argue that the recent attack with high S signatures was a major concern (quick fixed in 0.11.1) and IIRC seg wit is planed as hard fork in the long run.