But some of them are really expensive. I'd like to see inexpensive ones that simply allocate among inexpensive index funds, like the S&P 500, Russell 3000, US Treasury Bond, etc.
That will keep expenses low. The only thing the fund manage would have to do is keep the assets balanced, and adjust the allocation as the target date approaches.
I may be wrong about this, but I believe the best way to keep expenses low is to have a large asset base in the plan for the entire company — which means participation rates and contribution percentages are high, and most employees have been in the plan for a long time.