Texas is a massive state with a huge population and a vast amount of natural resources. The state and the world have exploited resources from Texas, and many Texans have made a lot of money or even a good living off of oil. However, they have to take the ebbs and flows of the oil market if that will continue to be the case. The state needs to diversify with new industries to weather such downturns.
Most states rely on something too heavily in order to keep their economy going. This was true of places like Detroit and I'm sure there are plenty of examples in the past. It's pretty common and it sometimes comes with undesirable consequences. Texas does depend too much on oil and I don't feel too bad for them either.
Yes, I think that the Texas economy does rely too heavily on the oil industry. I think it has recently gone down hill just because of it relying too much. The price of oil is going down like crazy right now and you can't rely on something that you aren't sure you will have forever.
Texas is a huge and varied state and has many different industries that contribute to the Lone Star State's economy. Oil is among the top of these industries but it is not as crucial to the economy as many believe. With growing tech sectors surrounding Austin and a growing financial banking sector in Dallas, Texas has more to rely on than just oil.