I agree with everything Hunker posted, and I sympathise with her too.
The whole issue of breastfeeding is so complex.
Take the "happy mum = happy baby" phrase. It only makes sense for something where you think really both alternatives are roughly equal. No one would ever say
"I know I ought to use a car seat, but I like having my baby free to crawl round the car and come up to me while I'm driving, and happy mum = happy baby."
Clearly that would be absolutely ludicrous. So obviously "happy mum = happy baby" isn't something that's always a valid argument - it's only valid sometimes.
On the other hand, I think everyone would agree that:
"My toddler would like the park or the duck pond equally, but I prefer the park, and happy mum = happy baby."
is a perfectly valid reason to choose the park. So clearly "happy mum = happy baby" makes perfect sense sometimes.
So then we've got formula feeding. Is formula the equivalent of 'no car seat', or the equivalent of 'the park rather than the duck pond for a trip out'? I think how you answer that determines how you see the "happy mum = happy baby" idea. Personally I think a baby who isn't getting breastmilk is getting the slightly riskier of the two alternatives - so it's closer to them not having a car seat than to a trivial decision between equal alternatives like where to go for a walk. (Note I'm talking about the baby's experience here, not the parents' decisions - I'm not comparing decisions, I'm comparing outcomes - it's a really crucial difference.)
If someone says "breast is best but happy mum = happy baby" that's usually a big clue that they don't, deep down, really think "breast is best" at all - it's usually a sign that they believe breast milk and formula milk are pretty much as good as each other - because "happy mum = happy baby" only really makes sense in a context where both alternatives are pretty equal. We just don't use it where we think the alternatives are very different from each other, we only really use it where they're similar.
I'm not saying how happy a mum is is irrelevant, of course I'm not - it is relevant - but it can only have the single 'casting vote', as it were, if all other things are equal, which actually they're not, by default, when breastmilk and formula are compared. That's why a lot of people find "happy mum = happy baby" annoying - it doesn't make sense to someone who doesn't see the alternatives as that close to each other.
Of course one situation in which the alternatives get more equal is if breastfeeding is going badly wrong in an unrecoverable way - that moves formula up in comparison, and then when whether to continue or not is being weighed up every day, one day "happy mum = happy baby" may tip the balance - but at that point you're not comparing the same things as you're comparing when considering how to feed before a baby is born, for instance. So again, "happy mum = happy baby" can have its place even if you see breastmilk and formula as very different and not as equals by default - their relative positions can change as time goes on.
Personally I think that given how many people wish to breastfeed at first, the easiest way to make most people happy would be to make it possible for them actually to fulfil that wish. I think some people don't realize how common ways we have of talking - ways that are used to reassure people who are past the stage of having a choice (the rose-tinting of formula to avoid 'making people who now formula feed feel guilty') feed back indirectly into our culture and healthcare systems in ways that condemn more and more women to repeat the misery of unsuccessful breastfeeding. Not realizing that link, they can't see any reason why anyone would mind that sort of rose-tinting, and conclude that if someone does, it must be because they want to criticise people who formula feed. But that link is there, and that's why people get het up about the language - it makes some people whose feeding decisions are in the past feel better, but contributes to more people feeling miserable in the future.