Actually, I'm becoming more and more convinced that what they need is to decouple progression and most of the significant advantages from the arena entirely, at least if they insist on limiting entry by population ratio. The arena isn't an aspect of the game you can just ignore. It's not just the Arbiter or general mission progression, but some very significant advantages in the great hall and the arena tier itself. Comparing the gold 4 tier bonus + fully upgraded great hall to not doing anything in the arena at all is comparable to playing the game without putting equipment on your champions. It's a big deal, and that's not even taking into account your access to the forge, tournament rewards, quests etc. Progression in the arena is central to your progression in general, the same way doing dungeons for equipment is.
Making arena ranks exclusive, saying that gold is not something everyone should be able to reach, is not sustainable. Bronze and silver are all kinds of screwed up with the current matchmaking, which makes its function as a ranking system fundamentally broken since your success has more to do with everything except the arena, but at least everyone has a theoretical chance to advance in it, provided that they follow the arbitrary rules of player power management (and even then, it relies on the assumption that not everyone will so that you can take advantage of it). Gold, however, will only get more and more saturated. Let's just assume the game survives for 5 years. What are the chances of a new player to ever reach gold when it's filled with accounts that have had 5 years to consolidate their power? Skill? Please. Skill may be a factor, but it's insignificant next to luck, with time and money being your primary way to influence how many opportunities to get lucky you have. There is simply no scenario in which a player of any amount of skill will ever be able to reach the top without a significant amounts of luck/time/money involved. This isn't the case in any proper competitive game. If you've got the hand-eye coordination and reaction times of a god, you will be good at CS:GO whether you've played it for 10 days or 10 years. Here, you'll forever be playing catchup.
I'm starting to think bot teams is the only way to go, or at least some other form of arena point inflation that will push players upwards (e.g. points for being too strong for others to pick a fight with you). I don't know if there ever were bots, I've certainly never seen any I could identify as such with any degree of certainty. Let's be real here, the experience would be pretty much the same whether you were fighting against bots or players, since the arena isn't really PvP in any meaningful sense to begin with. Note that this doesn't have to mean it will be easy to reach gold, you could theoretcally put in much scarier teams than what a real player can manage, just that there's nothing gatekeeping you from doing so provided that your skill/luck/whatever is sufficient. Even if I can't do dragon 20 for example, I know that if I get the right champions and equipment, I will be able to eventually. There's nothing saying that sorry, we've hit the limit for number of players allowed to beat dragon 20 at the moment, guess you have to wait for one of them to quit or hope you randomly pull that one god tier champion that'll let you push out someone else.
As for platinum tiers, well, I don't know. I still think the idea of a game like this even having PvP is stupid to begin with for the aforementioned reasons, but at long as it's not important to overall progression I don't really care.