A 16 inch MBP is not a viable alternative for someone who values the mobility and portability of the 13.
You are absolutely right, yet not everybody is choosing a 13" primarily because of the portability, you are able to see that, right?
For some it's just the cheaper alternative (to the bigger MBP).
To handle certain workloads, one would like to spec-up a current 13" -
and then the price actually becomes very close to that of a 16", which is massively discounted all over the place at the moment.
I am exactly in this position right now; the price difference between the 2020 10th gen 13" I'd like and an entry level 16" is negligible (the 16" actually being cheaper), so the decision what to go for is really about the differences which are so eloquently presented in that very video.
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It must be a pretty small amount of use-cases where both the 13" and the 16" both fit well.
I suspect there's more use cases than you'd think - it's just that the high price usually rules out the 16".
Now with the high-tier 13" being quite pricey and the 16" discounted as it is the situation is a bit different.
And, regarding the 16" being "not portable" -
I know for everybody who commutes to work by bike 300 days a year 1.4kg vs. 2 makes a difference, sure,
but for everybody else I don't really think it's such a big deal.
It still fits in a backpack and it's not like you're climbing mountains with it, right?
And you actually get 16" to work with in a form factor like that, which is really quite something.
Sure, if one likes small and light by all means go for a 13",
but people who actually need the machine for work will happily carry a 16" if it gives them what they want.
I used to carry 2-3 times the weight in equipment on a daily basis in my job for 20 years, just sayin'.