You have to put progressively more effort into making designs simple as there are more cards, but that doesn't mean simple designs have become impossible.
Similarly you can keep getting toothpaste from a tube forever; you just need to apply more and more force.
http://wondermark.com/776/Which is to say, I disagree. And I mean there's no way out, there's no, oh there's the secret corner of simple cards. For example, Smithy is +3 Cards. That's the end of the line for that level of simplicity; there's no compelling single + left to do. Whatever hope +4 Cards had vanished when I did +4 Cards with a bonus. I could potentially do one via having a new kind of cost like debt, or a new kind of + like +Coffers... but that's added complexity, I just moved it to the rulebook. I can't make the same argument for e.g. Harbinger's level of complexity, and there is probably stuff left to do at that level. But you definitely run out. It's not just a question of how lazy I am. There is some amount of complexity, that Dominion has already used, where you can keep going long enough to not worry about it; the goal though is to be less complex than that.
I will explain it in detail. Card text is made up of qualified rules atoms strung together by program flow. Rules atoms are the smallest things you can do in the game; qualifiers are the most basic ways of distinguishing things. Program flow is logic for doing things - "do a then b," "if x do a else do b" and so on. There are only so many rules atoms, only so many qualifiers, and only so many ways to have programs flow. For a certain number of steps, there is a finite amount you can do; it's inarguable. And Dominion then drops huge chunks of possibilities by avoiding having cards that are so similar that players wouldn't like it. As well as e.g. by avoiding cards that would just be stupid. It also has very few rules atoms.
It means you have to decide whether you prefer to spend increasing effort or prefer increasing complexity.
Those aren't the only options. There are spin-offs; there are unrelated games.