It's amazing how the snobbery leaps out of some people the minute they hear the words "Primark".
As others have stated, you're often not paying more to get clothing made under more ethical conditions or with more environmentally friendly methods. What you're paying for, when you buy that £45 "more upmarket but still high street" t shirt instead of the £5 Primark one, is the brand name. The product will often have been made in the same place, of the same materials, in the same way. In the case of a lot of these companies (Topshop, River Island, Mango, Zara) they're even sharing factories with the cheaper outlets you despise. They're not superior to them. They're just playing on your snobbery to make a profit.
I work in retail (not in Primark) and have seen the cost price on the items we sell. I think people would be shocked if they realized how huge the mark up is on some of it. Imitation leather handbag selling at £65 for example. Cost price £5. You're not paying for quality, you're paying for marketing, wages, and the biggie of all high street businesses: rents. Primark are cheap for the same reason Lidl are cheap - they cut out all the faff and buy and sell in bulk. That's it. There is no dark secret.
I think most of us who shop between the two know this. We've all got multiple items from Primark that have lasted years and wash very well, and we've all splashed out on something more expensive and had it turn to rags within a month. I've watched a friend try to convince herself discoloration of the fabric is just "a thing that happens sometimes" because her 100% cotton £50 Calvin Klein t shirt became covered in rust coloured streaks after one wash. (Not the machine, if you're wondering. All her Primark and assorted other clothing in the same wash came out fine.) She later went on to repeat the mistake with an even more expensive sweater, because she was so convinced that more expensive = better quality. It pilled after one wear. I'm not saying more expensive items aren't occasionally better tailored or nicer quality, but it really is the luck of the draw, in my view. Taking Primark out of my rotation and frequenting these other stores instead would only be making a mug out of me.
If the posters saying we should "buy better quality" mean we should actually buy really higher end designer stuff - £100 for a plain white tee or what have you - then they've lost all touch with the incomes of working class people, and all I can say is, I hope they're as passionate about implementing a living wage and guaranteed working hours as they are about saving the planet. Something tells me otherwise though.
I'd put the "shop in your well stocked local charity shop full of vintage designer gear and high quality off cuts" argument into the same woefully out of touch camp. If anything really nice makes it to any of my local charity shops, it gets "sent on to headquarters" ie, sold on to someone who will make more profit selling it abroad. Or shipped up to one of the classier big city stores with a reputation to uphold. What's left for us small towners? The stuff that's not worth selling on. You guessed it, Primark and fast fashion. Maybe some extra shipments of fast fashion stock the stores themselves couldn't shift or sell on to outlets like TK Maxx. Past season stuff, usually unsold because of how ugly it is or because of how low the demand for certain sizes was. It's not exactly the creme de la crème.
And you certainly couldn't just nip in to the charity shop if you wanted something specific, like a plain black jumper in a size ten, or a pair of shorts for your eight year old daughter. You'd be there for hours and still most likely come up empty handed. Not everyone has the luxury of that sort of time on their hands. For a working mum, popping into Primark on the school run is much simpler.
People may wrinkle their noses at fast fashion, but there's a reason it's such big business, and it's not just "shallow, frivolous women buying stuff they don't need". It's often women buying stuff they do need but can't afford elsewhere.
The vast majority of Primark clothing isn't made in China either, by the way. It's Bangladesh, Romania, or Turkey.