I don't fly SW at all, but I've flown pretty much every other airline multiple times, alone with a small child, in multiple countries, without paying for upgrades. Pretty much common sense that you don't seat a three year old next to strangers. This hasn't been an issue at all. It's not a problem, because no one wants a middle seat.
I guess the guidance is just saying something the airlines already want to do, or putting an actual age limit on where they should keep doing this. Or, I can see this being problematic with more than one kid, which I haven't done.
I can also see Southwest being a problem for this, if you don't get the priority boarding, though.