(for a 70% increase in capacity - hmm).
That's incorrect. Segwit does not make blocks 4 MB but only allow for a 70% increase in capacity. It almost doubles the transaction capacity and also only doubles the absolute size of a block.
I am sure you are right. In the worst case a block can be 4MB, but in reality that only happens with malicious/unfortunate input. In average, it gives almost a doubling of capacity for a doubling in data size - at least once it starts getting used. Which is an important first step in getting bitcoin back on track as a digital currency.
There is a tendency to judge algorithms and the like by their worst case performance, while it is usually the average performance which counts - unless there is some reason to think the worst case will happen often in real life.