Just adding my thoughts here. The ranking system seems pretty good except for the part of demotions. I feel you SHOULD be able to drop between tiers.
Reasons:
1) it might encourage trolls and hackers to get to a high rank and then throw games to prevent others from reaching the ranks. I assume you will have a quitting penalty but what about those who throw games ?
2) it adds more meaning to games played towards the end of the season, no matter of your rank. Based on your numbers, a month will be more than enough for a highly skilled player to reach Apex predator, what will they do with ranked after that? No matter if they win or lose, then have Apex predator so why bother?
3) Dropping between tiers makes things more challenging as well - your grind doesn't end the second you make it to the tier, but you gotta keep working to stay in that tier (similar to the style Overwatch uses). Players make fall back a few times as they work on their skills to match players of a high ...