When you hit Tier 8, it stops being so profitable.
The premium ammo will always stay, its how they make money, I think they made it possible with regular credits simply because you couldn't compete in clan wars unless you were running premium ammo.
Plus the fact is, if you own territory, you get gold per day, so if you pay for the ammo, you get the territory, which gets you the gold, which means you'll never lose and not have to pay for gold ammo anymore eventually. It had to change. It sucks up until tier 8+, because it gives a massive advantage.
And yes the physics are ace, I had a laugh on my first game with it. I used to just cruise control until the cliff edge stopped me, until I fired my light tank off a cliff into water