Wouldn't Austin, Houston, Dallas, and San Antonio vote it down?

That assumption is built on a stereotype, not on the numbers. Support for Texas independence is broad enough that it does not live or die by the big cities, and the cities are nowhere near as one-sided as the caricature suggests.

The polling crosses the whole state

Surveys have put support for independence around 60 percent statewide, with about two-thirds of Texans wanting the chance to vote on it. You do not reach a number like that by running up the score in rural counties alone. Support on that scale necessarily includes millions of people in and around the major metros. The picture of a rural Texas straining against four solidly opposed cities is a cartoon, not a measurement.

Cities are not monoliths

Houston, Dallas, San Antonio, and Austin contain millions of conservatives, independents, small-business owners, working families, and first-generation Texans who are exhausted by Washington. A city that leans one way in a partisan election is not a city that votes as a single block on the question of whether Texans should govern themselves. Independence scrambles the usual lines. It is not a normal left-right vote, and it will not break down along normal left-right maps.

Independence is not a partisan transaction

The case for self-government is not "your team wins." It is that decisions about Texas should be made in Texas, by Texans, close to the people they affect. That argument lands in a Houston suburb as well as it lands in the Panhandle, because property taxes, the border, the cost of living, and a distant unaccountable federal government are not rural concerns. They are Texan concerns, felt hardest in the metros where most Texans actually live.

A statewide vote is won statewide

A referendum is decided by a simple majority of everyone who votes, everywhere. The job is not to win a region. It is to make the case to every Texan, in every county and every city, the same way any winning statewide campaign does. With support already polling well above the line, the path does not require beating the cities. It requires turning out the majority that already exists, in the cities included.

The bottom line

The big cities will not sink independence, because the big cities are full of Texans, and Texans, by a clear majority, want the right to decide this for themselves.