Oh, there's no doubt that Platinum League and even Gold League will probably be tougher fights than FGs -- I'm just saying so long as we make the rewards commensurate with the risks, that's a GOOD THING.
*shakes head*
Imagine for a moment that T500 has been here for a while, and the veterans (mostly Platinum level people) had the time to get used to it. Imagine that T500 Platinum League rewards are VERY intresting (i.e. all cards upped, five spins, every three fights you win an automatic rare card, which is quite possible). So, you want to farm T500 Platinum League.
What will you expect in there? Fully upped decks with a powerful strategy, many hp, fully functional for the AI to play, fast versatile and deadly. I'm expecting a mix of rainbow rushes, poison stalls and destroyer rushes, possibly some other intresting decks once in a while. No matter the 5 cards rule, you can almost always create one of those archetypes.
What would you bring? A control heavy deck with purifies. You need fast control, otherwise destroyer rushes will kill you; but you also need repeatable control, otherwise rainbow rushes will kill you. You need to go rainbow, otherwise you haven't got enough options, and you need an implacable assault (like neurotoxin) otherwise you're losing to control decks.
The problem is that rainbow rushes can pack any kind of card to answer to your strategy, and are a whole lot faster than you'd like them to be. You try poison, they have purifies; you try rushing they have 200hp and miracles, you try controlling, they have massive PC, you try Black Hole they have more quanta, SNovas and Sanctuary.
Destroyer Rushes can quite probably get 4 turns wins reliably. If you try to control them, you'll probably end up losing to their double draw alone: you can't keep up.
Poison stalls will be the easier sort... except that they can be REALLY fast. Try going against an FG-level Speed Poison: you'll end up having more than 30 poison in about 6 turns. Also, they will pack defenses that almost eliminate your chances of victory. A False God with 12 Shards of Gratitude? Come on!
You end up with the following conclusion: trying to outstall them is quite frankly impossible, so you have to rush them. Your win rate will be very low, but at least you'll win against bad draws. Da Fastest Speedbow Eva can kill FGs on a good hand, and I'm sure it can be tweaked. Except that when Platinum people manage to figure this out they will add counter cards in their decks: heavy CC, BHs, 12 rewinds... there are endless possibilities. They will sacrifice speed for that, so that I will be able to outstall them, you say? No. They will just switch around a couple cards, since in a 60 cards deck it is rarely a problem.
Really, Platinum will have the lowest win rate EVER. No matter how much the rewards are, you're just going to lose too many duels, forcing you to speed farm for it to be worth it, making it really easy to be hard countered. How fun is playing Octane with RoL Hope? 90% of the times you're going to end up with that kind of duel...