This is all speculative, so I cannot say for sure how the Arena/Season system is going to work in regards to moving up the tiers in each division. This is solely my analysis/concerns with the information that has been given.
The issue at hand is the length of the Arena Seasons. It has been announced that the Seasons will last 1 Month. When examining the style of tier/division system that they are using, and comparing it to other games that implement a similar system (League of Legends/Smite), I fear that the time given will not be good enough to really gauge a players true skill.
Here is the number one reason why.
Lack of Play Time - I have done some relatively rudimentary math, and have come up with these numbers as a simple base to start with. Assuming (Speculative) that it takes 10 wins (0 Losses) to move through a given tier (Silver 5 - Silver 4), one would need to play a lot of games (300) with a 100% win rate to reach Onyx Division, and compete for Champion. 5 Division * 6 Tiers would mean that you would need to average a tier each day for 30 days just to enter Onyx Division. This is assuming that you place in Bronze 5 during your placement matches.
The problem that I am finding is for those of us who may be Onyx Division level players, but aren’t necessarily good enough to win every single match all the way up to Onyx Division. For example, if you have a 70% Win Rate, this would mean that you would (in a perfect world) win 7 games, and lose 3 for each tier. This would require you to play a total of 13 games for each tier to make up for those 3 losses. So now the total of games played (perfect scenario) would be (390 Games). This number becomes exponentially larger for those that don’t average a 70% Win Rate.
My point is this, if you average a 70% win rate, odds say that you will move up through the ranks and ultimately reach Onyx Division, just through grinding because the system is purely based off Wins/Losses. You are good enough to reach Onyx, but you had to fight your way to the Division, but it was still earned fair a square. If the seasons are only 1 Month long, this could potentially alienate those people who have a decent win rate but a lack of time to play all the games necessary.
Anyways, this is all skeptical, and obviously requires you to be good enough to play your opponents at those tiers/divisions in order to secure those wins. I just would hate to see people miss out on their true skill/division based purely on a lack of time.
TL;DR - There are 4 Seasons in the year, why not make each Arena Season 3 Months instead of 1 Month.