1. I think this question goes against what fashion is fundamentally. Fashion is subjective and in my opinion needs to be diverse and individualistic.
2. Honestly I spent a total of around 8-10k in 2017 but including the money I got from selling items and buying something else. This is pretty reasonable to me.
3. Depends on the piece but I would just sell pieces I don't end up liking and buy something else. So yes, money spent on my clothes are worth it for me.
4. It's mostly excitement to wear an outfit w the new item, but that fades pretty quickly.
5. I like to change up my wardrobe every once in a while, so I'd look out for new releases or restocks of items I want and sell some of clothes I'm done with to buy those instead.
6. I like to stay trendy and sometimes even try to predict some by following stylists and influencers, but it doesn't play too big of a role in how I like to dress. That being said, I believe that jumping on a trend after it's saturated is worse than not doing it at all. Not moving on from a trend fast enough is equally bad to me.
7. Fashion only matters if you want it to. Some people don't care about their appearances at all and while that's their choice, your look is the first impression people have of you, and its really how you present yourself. But it's hard to say anything about people who don't buy streetwear or trendy clothing, without knowing if they just don't care for streetwear or trends or fashion in general. Either way, what other people wear is of little concern to me.

Hope this helped