I guess I am a bit more of a casual player than most at this stage. The arena key system is a bit rough. Only level 50 at the moment, so I might not have the full gist of it yet though.
It appears that you need to complete 5 maps or so before the keys will start dropping. Dying resets that timer as does logging out.
Generally speaking, I only have a few minutes to play at a time so I might only run 2 or 3 maps and then log off. I have resorted to leaving the game up so as not to lose my streak so I can still get keys.
I don’t think that there is something inherently wrong with the current system, but it feels like it penalizes me for playing the way that I am able to where keys would be super rare for me. I think I have died in maps twice so far, but other than that I only have gotten two keys. Both yesterday and because I left my game running for several hours until I was able to complete the 5 maps. I ended up getting two keys before things were a bit rippy for my character and I lost my streak.