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GGG doesn't know how to "slow the game down". They never did in POE1, and still don't in POE2. Their answer to combating fast gameplay is to nerf skills, add more tediously unnecessary gameplay (boss phases, levers, increase zone size), and make the monsters faster/stronger.
The core of your post revolves around the idea of "all you have to do is just buy your next item from someone, and that makes the game easy". You going on trade and buying your power items just to say that the game essentially needs to be harder is a problem you're putting yourself in. If you want the challenge you seek, play SSFHC. You'll find it's a drastically different experience. The game forces you into that slower gameplay because 1 mistake can and will send you back to character creation. Yes, the same overpowered builds exist and are viable in SSFHC, but getting to that point is
far more difficult and time consuming than it is when playing in trade.
The thing is that they don't seem to even try to do anything that way...but regarding SSF I get what you mean, I will go that way when we will have more skills and characters, I can't wait for it.
Only thing that bothers me here is that there is no sort of league where you can pick SSF only in a party and you stay with that party of 2-3 people, so if you want an SSF with friends, you enjoy that journey, but when you're an archer, and your friend drops a bow, he could give it to you instead so you could play like Team Found or something like that - Seen that on one private WoW server with hardcore setting and that works amazing.