Partly, yes that is a mistake. The importance of diversification is that, let's say you're invested in American large stocks for the next 10 years. Let's say it trades sideways for that entire time, but BRIC does outstanding, emerging markets does +8%/yr, and EU has a crash over the next 3 years(hard crash) but then start's going on a run for the next 7, but you invested only in US Large cap because of the expense ratio.
You missed out on all of those positives in other markets by putting all your money in only one thing. Yes, expense ratio is incredibly important, but so is diversification. Example: during the .boom it didn't matter what tech stocks you had, every single one of them was going to pay you handsomely for your time up until the bust, where a lot of them fell sideways, some disappeared, etc. Another example that Peter Lynch cites from his own experience was US Auto stocks during his time managing Magellan. Sure, he picked the better ones by going with Chrysler(+300%) I believe, but the others still doubled in value at the same time.
So, while .07% is definitely something to put decent chunks of your money in, Markowitz's portfolio theory shows us that by diversifying our asset classes we truly can gain from it, and though 1% for a targetted date is fucking stupid as far as I'm concerned(put in your own leg work and balance it yourself), .3-1% to get exposure to other markets/asset classes is something worth doing. And when the shit hits the fan in the US or Europe, it's unlikely that the same thing is happening for the same reasons in Brazil, or China.