I think the arena became so difficult completely organically, as part of natural progression of the game. And, sadly, as time passes it is likely to become even more difficult. Here's how I think it works:
A lot of Platinum players are OG, they have been playing for 2+ years, some even since the game was released. As such, they had a lot more time to farm god-tier gear, spent more on getting the meta champs, fine-tune the teams, etc. Once they reach certain level, they don't quit the game, but prefer to stay there.
As a result, the top of arena gets filled, saturated, so to speak. As newer players arrive, they occupy slightly lesser positions, but are still competitive for top postions.
Then come even newer players, with less than 2 years worth of playing. As Arena if filled from top down, those players are at a huge disadvantage, because there simply wasn't enough time and gameplay to progress their accounts enough to catch up with the rest. One way, if not the only way, is to spend far more than OG. This is why it becomes more and more difficult and more and more is required to spend money to stay competitive.
The addition of G5 was a great idea - but only on paper. Instead of staying in G5 or pushing for plat, those ''plat dropouts'' as i call them, drop down more or less on purpose, some want to farm more easiliy, others with maxxed or near-maxxed GH simply don't care. They're not in any rush. So they keep dropping to G4, G3, or even further, and are in the way of others who are struggling to reach higher tiers.
The problem is that the arena is saturated from top down, and as time passes, it will completely saturate G5 and eventually G4. SO you will end up with players trying to get to G4 to get Arbiter, and G4 will be filled with people who have been playing several years. And this is what they'll be up against.
One solution to this disaster-in-the-making is to EXPAND. EXPAND EVERYTHING! Plat, G5, even G4. This is absolutely paramount. Otherwise, arena will become completely unplayable for any new player that comes into fray within a matter of months. I'm referring to G4 in particular. It will become impossible to reach, even for high spenders. And when I say ''expand'' I don't mean by 100 or 200 places - but rather 1000 or several thousand.
The other solution is to un-tie game progression (get Arbiter, get Ramantu, get MyLateGrandma...) from reaching G4.
I am pretty sure even the brass and the developers at Plarium are aware of this looming problemm which is only going to get worse with time. The last thing any company, any business needs is to demoralize their potential customers. That's the best way to lose them. For evidence, i present the OP.
I hope i made sense. I would write some more, but i ran out of ink.😁