I know a lot of formula feeders in "real life" (and I know a lot of breastfeeders too)
From general conversations etc, I dont actually feel any of the formula feeders believe that formula is "as good as" breastmilk. I think they all accept the message that "breast is best" at face value - i.e, breastmilk is nutritionally superior to formula milk.
They realise this, but they still formula feed, or supplement with formula, knowing this.
Apart from individual reasons for doing so that aren't nutritionally driven, I think its more of a case that they dont believe it makes so much of a difference that it will directly and strongly impact their baby.
A cereal bar is nutritionally superior to a Milky Way bar. A Nairns organic oat cake is nutritionally superior to a McVities Digestive. A corn fed steam fried chicken breast as the main component of a hot meal is nutritionally superior to Birds Eye Chicken dippers as the main component of a hot meal.
Yet, children will be served the nutrionally inferior version from time to time, or sometimes all the time, because its not considered to be so important as to warrant the feeding of the nutritionally superior version at whatever cost, be it financial or practical or otherwise.
I suspect people's opinions on the above fall broadly into two groups. Those who believe this is very sad and that things need to change to educate mothers of just how nutrionally superior breastmilk is, and those who feel that the benefits of breastfeeding are at best overstated.