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. 

 what post were you referring to here? I don't see a reference in Primal..