This could be compared to nerfs to PVE instances overtime.

Blizzard put resources into a arena set for example, or PvP set if you would like to call it that.
Now, only 5% of the communitybase (values made up obviously, just for example) get to get full set.
This leads to a problem for blizzard since they want as many people as possible to experience the game they have created.
In this case, i think its a good idea and you can compare it with the PVE part since, you will have your full gear in the end of the season = nothing unfair.