If those jeans cost less than 20 euros, how much is the person who made them getting? Environmental harm isn't all about fibers and dyes. It's also about keeping people poor. Poor communities have no other choice than to pollute with sewerage, chop down forests to heat with wood, pollute the air if coal is cheap, trade or consume endangered animals, etc. That's why organisations like Jane Goodall are so concerned with supporting communities near the areas they want to conserve.
I agree - the conditions and wages of the workers who made the clothes, usually in poor countries, is a big concern. Especially after the Rana Center disaster in Bangladesh a few years ago. https://nordic.businessinsider.com/heres-how-much-hms-clothes-would-cost-if-factory-got-paid-sustainable-salaries-2017-6/
That's true, but it's not like they get paid more when the clothes are more expensive. The difference goes into the pocket of the merchants, not into the pockets of the ladies who sew the clothes.
The problem with ethical clothing is that there are several aspects to consider: social aspects (like child labour) environmental aspects (like the use of harmful dyes and treatments) and economic aspects (fair trade). I am not aware of any brand that does well on all of these aspects.
There are also many different accreditations for conscious fashion. A company like H&M is FLA accredited, which means their factories meet the FLA standards - that honestly means they're just slightly better than other factories in the same country, it's not what we would consider normal working circumstances in the western world. That doesn't mean there's fair trade, which is almost impossible to achieve for a large first-world company getting their supplies from third-world countries. There are several certified organic accreditations that a company can achieve, but just because they are using organic fabric, that doesn't mean they treat their workers well or there is fair trade.
It's not like I haven't done my research into this. I think the most important thing is to try and buy secondhand, but I know that's not always possible. The second important thing is to wear all your clothes until they are worn out. You shouldn't buy items that you're only going to wear 5 times. I wear the same times for years and years. The third important thing is to have the smallest amount of clothes possible. Not all clothing is easy to find on the used market. I think about 75% of my clothing is either secondhand or homemade, but about 25% is not. For that 25% I have no problems going to stores like H&M.
I only buy quality pieces that will last for years, I've done the research, I'm trying to limit my ecological footprint as much as I can, but I also need some things to wear. That means I will buy maybe 2 or 3 new pieces of clothing on a yearly base and I'm ok with that.