I watched the 2011-12 episode of the Premier League Years on Sky t'other day. One thing that struck me was that outside the top 6 (which is the same now as it was then) every single club in the league bar Everton in the above list has been promoted from the championship at some point in the 6 seasons since. Think about some of the teams we beat that season: Wigan, Blackburn, Bolton, Villa, Wolves, Fulham and of course QPR. Sunderland took four points off us that season (more than anyone else) and look where they are now. So of that 2011-12 season, 12 of the 14 teams outside the top 6 are no longer in the premier league. The others are Everton and Newcastle, who of course have been relegated but have come back up since.
The top half of the Championship tends to be populated by clubs that have been in the premier league recently. Sometimes they are yo-yo clubs like Norwich and QPR who are down for a couple of seasons, up for a couple of seasons and then down again. Sometimes it is a club like Bolton who have been in the premier league for a while until their bad season came along and they ended up getting relegated and now struggle to break clear of the pack. Sometimes it is a club Every now and then you get a team that goes into free-fall after it is relegated, like Blackburn, or Wigan or Sunderland, but mostly the relegated clubs come back up within a few seasons. (Even we did that.)
So we have a top six where you would expect the four or five teams in the Champions league to progress from the group stages into the last 16 without too much difficulty, and a bottom 14 where all the teams are pretty much at an equal risk of being relegated either this season or next. When you consider the fact that the newly promoted clubs tend not to go straight back down, three things seem clear to me: (1) there is not actually that much difference quality wise between the bottom half of the premier league and the top half of the championship (2) Every team outside the top 6 is highly likely to go down at some point in the next few years, with the possible exception of Everton, and (3) Our top 6 is the equal of any top 6 in Europe and collectively is quite possibly the strongest in Europe.
So when the OP says he was disappointed this (17/18) season with the quality of the league outside the top 6, I think he needs to get used to disappointment, because no matter who has gone up and who has gone down at the end of the season, the quality of the league next (18/19) season outside the top 6 is likely to be no different to this season.
And since every team outside the top 7 (including Everton) spends two or three years every now and then outside the money zone, that means the gap between the teams that never get relegated AND get European football every year, and the teams that get relegated every few years (that is, all of them bar the top 7) will steadily get wider and wider.