25 shards for the best of the best over a period of 3months seems legit.
So after playing (and be on top of the leaderboard) for 3.5 years you have at seven stars.
If you are top 50, it takes you 5.5 years...
That's a looooong tournament, but the devs have enough time to adjust the skills to match the power level to 2020s characters.
Well, I don't want to go to deep on details, but I would suggest not to touch the current arena design and for seasons, just add another game mode (PvP).
I would still prefer more PvE Content. That's fun if balanced.