Yes, you can. You are right that this is actually a riskless trade, but the profit is rather minuscule. The contango used to be 10-12 ES points for 3 months, now it is only 3-4 points and sometimes it is backwardation.
So yes, a retail trader could be long the index and short the future locking in 3-4 points monthly profit, but the ROI is small.
Edit: This just gave me an idea:
If I do a covered call on the index while I am short the future, that increases the riskless trade's return. Even in there is no contango, the option premium will give me a return and I close the future trade when the stock is called away, if the calls are in the money.
A quick calculation with commissions subtracted gives about 6% annual return. Hm, that sounds to be too good for a riskless strategy...