Rainbow decks is no longer a winning strategy, but they will also be the only decks being played.
How are people going to play just rainbows if there are no rainbows posted?
Also, I gave one example of a deck that people can bring. Other people can bring in decks that counter others. So in the end, rainbows are no longer a winning strategy because of people bringing in black hole decks. Mono decks that are not particular fast is no longer a winning strategy because of other people bringing in discord-black hole decks optimized to crush said mono decks. Few/no PC decks is no longer a winning strategy because still other people will bring in instosis/mono aether/etc. that rely on permanents to win. And so on.
So what you're saying is that there will be a variety of decks and that Arena strategy will evolve very quickly in response to what decks people are playing? This is a good thing, surely?
Maybe if I put it this way, my argument is clearer:
zanz implements your suggestion. It's been a week/month (basically some time has passed so people have adjusted). You're a deck creator. What should you put in?
1. rainbow - the only people who would play it are those who brought in black hole decks; everyone else sees the deck list and decides to skip it
2. no PC deck/life rush/etc. - the only people who would play it are people who brought tons of sundials, dim shields, perms (that help survival); everyone else skips
3. some other deck - people who play it brought in the appropriate counter deck; other people skip
4. some strong deck that has a unique counter - everyone skips, you get no plays
The above should encompass all possible decks you can build. If you build something in 4, then your deck will fall out pretty quickly, since you've said that decks that don't get played will rank low. If you build something in 1, 2, and 3, you get some wins, but probably mostly losses. However, since they get played, they will still rank higher than decks in 4. So you're given the option of putting in a deck that will lose often or a deck that won't get played. What do you do?
My opinion is that eventually, deck creators will get tired/demotivated and stop trying, and the arena becomes a joke. You seem to think otherwise; if you can, care to explain why?