IMO, the Endless Arena is a really niche thing that there isn’t much point in doing unless you really care about leaderboards. Getting Arena Keys as echo rewards feels like a waste of a reward slot that could have been something - anything - else.
I think it would be better if these were acquired in a different way.
If it was just a key-node that had chances to drop every key it would be a lot more valuable. Would probably run it most of the time it would end up in my path.
Echos should always reward 1 key (random selection), and remove them from the random drop tables completely. Then, remove the key reward-type from echo completion as well.