It's not only that you have 1 season to get, but most of them are just for a small % of players.
I think in the more than 15 years that I've been playing, I've seen like 3 different people with a gladiator mount, and I don't even remember seeing ever anyone with the "elite" armor sets.
And I agree, it would be great it works something like the Mithic mounts: you still can get them, but you have to be extremely lucky and dedicated.
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No, the equivalent would be to not able to do get the mythic raid mounts and the mythic raid sets once the next season starts.
Or make every cosmetic thing from an expansion unobtainable when the expansion ends.
The question is: Why is there that difference between PvE and PvP rewards?
Why someone can go and farm a mythic raid from 6 years ago and get the same rewards as someone doing it at its time, but they can't do the same with PvP?