Very good analysis.
I'd like to explain a nuance which some might miss.

When Ben says here 'it was never expected that blocks would reach the 4MB limit', he's *not* saying 'it was never expected that blocks would become permanently full'. The size of a full block depends on the kinds of transactions people are doing. If normal payment txs, it might be around 1.7-2.5MB, say. Only if you have weird txs with hugely bloated witnesses, do you have the possibility of creating blocks with close to 4MB total size.