With how rating works, your deck's win-rate cannot drop below 85% (5 wins before each loss) if you want to be above the 0-0 people.
The highest-ranked 0-0 guy is around 200. The lowest 0-0 guy is around 270, and below that are the 4-1's, 9-2's, 14-3's, 19-4's, etc.
Then the 3-1's, 8-2's, 13-3's, 18-4's...
Then the 2-1's, 7-2's, 12-3's, 17-4's...
Each time a player builds a new deck with a new oracle card, they bump down everyone ranked below the last (oldest) guy with 0 rating.
Now, let's say that you get 2 losses off the bat, but still survive because "each deck gets 5 plays." Where, in the 500 ranks, would you be? If you're at 250, then you're pushing out people who have 5+ matches and don't get "invincibility", such as the guys who have 8-2, 13-3, etc. How is that fair? Or maybe you're outside of the original 500...but that would mean that there are more than 500 decks in Arena, and so a player wouldn't be fighting the top 500 decks but 500 decks plus 200ish decks that should be dropped but haven't played 5 matches yet.