It's not the first time Houston has seen its economy decline due to low oil prices and it won't be the last either. Oil & gas is notorious for its boom and bust business cycles.
UA is just reacting to the fluctuations in the economy and I have no doubt that contractions in seat capacity are all but inevitable. No reason to think that the sky is falling however.
If they're smart they'll take the opportunity to diversify their operations and spread the fleet around the other hubs though. Record low oil prices all but guarantee increased profit margins for all carriers in the next few years.