"Without oil, the government won't have a source of income independent of the people"
You do not understand the "third world" socialist mentality. It's not how much you have it's that you control whatever you do have.
Mexico is in no more danger of getting less corrupt, less competent government or more freedom and economic development with oil than without oil.
If they had and respected a system of laws and property ownership, what you say might occur, but without a legacy of ownership and freedom, all the decline in oil production means to the average mexican is there will be less luxury consumed by the ruling class.