So your argument essentially boils down to: "It is only easier to read when developers write correct HTML." That isn't as "gotcha" of a rebuttal as you think it is.
It is easier to read in the common case. That's more than enough.
PS - Also a lot of IDEs will flag <picture /> with "missing closing tag" or similar.
No, my argument is that / is completely useless and is a false signal in HTML, it really is just ignored. I never argued that it makes anything easier.
An unclosed <picture> doesn't automatically become valid, but at least there's nothing suggesting that it can be done.
It is easier to read in the common case. That's more than enough.
PS - Also a lot of IDEs will flag <picture /> with "missing closing tag" or similar.