There will be always players who downvote just because they lost and would upvote if they easily win. I dont see why the first is no problem, but the latter is. Just ensure, that decks that loose everything still drop out quickly.
However for decks that just do 50:50 (which is clearly subpar in Platinum): These are the decks that should be able to survive for at least a while. The top decks in Plat are something like 40-0 or 60-1. Congratulations - if the whole league consisted of such decks it would be completely pointless and utterly frustrating. In the subforum for platinum decks you can learn that even 70% winrate is considered rather mediocre. If the league had only decks with 70% to 100% winrate, the average rate would be 85%, or 15% for the player. This is surely below the frustration threshold for any player (which I estimate at 30% mininium, perhaps rather 40%). Conclusion: The beatable decks are those who keep the league attractive.
The monoculture from player side is difficult to break because the few named decks are the ones which lift the player winrate above the named limit which I called frustration threshold. It depends on your personality of course, if you are satisfied with 20% wins you can do a lot, but if you want 40% there are hardly any options. I tried a bit with my rainbow deck could not find a variation that even came close to that.
Essentially the situation seems to arise from the fact, that everyone want to be successful:
1. Players want at least 30-40% winrate
2. Deck builders want 80% winrate
This contradiction leads to the ruthless optimization from both sides. You can't do a lot about the first point. First, only winning gives positive feedback in form of cards and coins. Second, the threshold is psychological one - hardly anyone likes to loose the vast majority of games, even if 20% would still gain money. Hence, the second one is the point to change: What deck builders consider as success. Currently, there is only one benchmark criterion displayed (and really relevant): Ratio of wins and losses. Of course deck makers will look at this criterion and try to be the best in this regard, which means in Platinum aiming at close to 100% winrate. So the solution is: Offer a different measurement of success that incorporate other qualities. The variety in player decks will follow, if the pressure on the 30-40% threshold is relieved a bit, because using always the same gets really boring.