Saying "the corn will be grown anyway" is not done in good faith. The corn is grown because it is profitable to do so. Remove subsidies and the corn will not be grown - something else that is profitable will be grown.
The trouble is that it's unpredictable how big the crop will be before planting. As mentioned before, yields can vary quite significantly from year to year. As such, you will have periods of excess where we don't have enough storage for it all. Ethanol was promoted as a way to clean up those excesses before they turn to waste, recapturing some of fuel that was put into its growth initially.
[1] - https://www.ers.usda.gov/topics/crops/corn-and-other-feed-gr...