"They have space in their lives for a cat, so wouldn't it be better to take one out of rescue rather than pay people to bring yet more cats into the world."
Hmm, my first cat was a Siamese - I actually didn't particularly want a cat, my DP did and she was our compromise between a cat and a dog... I've now got a moggy (they did co-exist for a while, but the Siamese is no more)
My moggy is lovely and I'd not be without her, but I have a Siamese space in my house, not a cat space - they're not the same at all.
If I was looking for another cat, rescue or otherwise (and yes I'd always look for a rescue first) I'd not be looking for a moggy, not because there's anything wrong with one, just, they're not the same type of pet at all.
There are different types of breeders, I'm ok with giving money to someone who does everything they need to do for the health of their cats and is trying very hard not to add to rescue populations by making sure they stay in touch with the owners they sell to and having an open offer to always take back a kitten they produced, some breeders are involved in rescue as well.
That's a hugely different situation to someone who is trying to make a quick buck selling badly bred pedigrees on gumtree or someone who just leaves an unneutered cat to have kittens with whatever tom it meets whenever it goes into heat and then sells them off.
It's definitely different from intentionally breeding cats with a cartilage problem because it looks 'cute'.