I'm probably in a minority that doesn't see much use in 20 guage guns being that the wide variety of available loads in 12 guage will allow one to do with it what can be done with a 20 guage and more.
I understand where you're coming from and you're certainly not wrong. The 12 gauge surely will do all that a 20 will do and more.
However, we could apply that logic to every caliber.
Why buy a .30-30 when a .308 will do everything and more.
Come to think of it, why buy a .308 when a .30-06 will do everything it will do and more.
Thirdly why buy a .30-06 when a .300 Win Mag will do all that and more.
You know what, why buy a .300 Win mag with a .300 Ultra Mag will do all that and more.
You could do this all day.
We like certain calibers and gauges because maybe that's what we grew up with and we're nostalgic. I have a soft spot for 16-gauge because that was my very first gun. An old hand me down Winchester 37 Steelbilt single shot. I'm the 3rd Paul in the family to have it and back in the day it wasn't used for pleasure, it was literally used to put food on the table and if it hadn't have, my dad's family would have been vegetarians.
There certainly isn't anything a 16 will do that the others won't. It doesn't even fill a gap between the 20 and 12. A 3-inch 20 gauge round will do everything the 16 will. Heck most 16s are built on 20 gauge frames so they kick as hard or harder than a 12.
But I sure smile every time I see one and I feel all fuzzy inside every time I raise that old single shot to my shoulder. And that's enough.