A graph of world economic growth since the invention of fire shows anything but diminishing returns however. If rent-seeking behaviour is increasing, it's because the pie is - at least on a global level - growing at an accelerating rate.

Elites always have and always will seek to take disproportionate shares of that wealth because they can, and because regimes, legal norms and competition encourage them to do so rather than because creating wealth doesn't pay. Much of postmodern finance is undoubtedly rent seeking, but so were the fights over ownership of natural resources and sovereign-granted monopolies of earlier generations, and the local nobility and religious bodies taking the surplus agricultural produce before that.