I think there a few reasons for that.
This new system doesn’t have a ceiling, so there’s a little more room for lower skilled people to fit into higher ranks. However, even though this system is a little easier to exploit for rank purposes, you still won’t stay in a rank if you can’t actually hang with a higher level players and stay alive.
Whereas gears 4 had a limited number of slots in diamond ranks (not sure about onyx). I can’t remember if they changed that, bc I remember loads of people complaining that many players would stop playing after hitting diamond so it was extremely difficult to break into diamond from onyx because this, even if they had the skills.
Three big issues with gears 4 rank system was that, number 1, some people should have been a high rank but weren’t due to limited slots, but on the plus side that would certainly keep the competition tight.
Number 2, was that people seem to forget you could get carried to diamond as well bc wins were a major determining factor in ranking up. I’ve played with and against some diamonds that were duds in gears 4 (back when gears 4 was most active and recently).
Then number 3, it did not do a good job factoring how good of a player you are currently (and no I’m not talking about your beastly game of the day). They took your playing history too much into account for determining your rank. You could have had a season or two where you sucked, but then seriously improved as a player and not be able to rank up without an insane grind because of how you played two season ago, even if you currently play at a fairly high level. Then you can have someone who was good starting out and gets better as well, but there is not the same level of grind to get to diamond if both player examples are playing at a diamond level.
My experience with the new ranking system this op is that your rank does a better job representing your current performance. Though coming back to the point of the OP’s thread, I think I’d agree about the current diamond being the new onyx. Or at least a hybrid of sorts.
As a side note, I’ve found once I hit upper onyx in tdm, a lot of the team matchmaking became much better balanced (evenly matched) and a lot of my matches have had many same/similar ranked players in most matches. I found it to be a lot more enjoyable because people generally have a better sense of how to work with a team, not waste lives as much, etc.