This is a thought provoking post. I am a new user and I agree with your points regarding spam commenting and spam post. Steemit platform is full of it. I must confess I am guilty of this myself sometimes. But the real question we should ask is why people are doing it? Let me address in a few points:

1. You started in Sep 2017, I started in Jan 2018; you know this already how incredibly hard it is to get a upvote or comment. Most people are doing it for the money (read easy money) and the system rewards for spam (read volume, not quality) at least to some extent;

2. I got to be careful with the next comment here; but the reward system is very much pyramid like and it is rainbows and unicorn at the top and fairly dark and depressing at the bottom;

3. Don't get me wrong, I think this platform is a wonderful concept but it is so hard to keep it clean; just think of Facebook and Twitter in terms of content; there is still great content there, but man ...the junk you have to deal with! 4) Many people here are not writers (including me) they just suddenly found out writing pays, commenting pays! Bingo!
   I personally stumbled into steemit following a youtuber who I follow regularly. I am here for the content. I am finding excellent content here. But we all must actively flight the junk.

PS. I will self up vote this comment, but is it the right thing to do?