Dallas County is lost for the GOP, but that's not a big deal in and of itself. It doesn't do much good to shut a party out of a county when that county's population growth has been flat for the past decade or so and the counties surrounding it are exploding in population and are very favorable towards the GOP.
The Democrats are going to have to figure out a way to get more than 35% of the vote in the collar counties (Collin, Denton, Ellis, Rockwall, Johnson). Same story in other "sprawl counties" near Houston (Montgomery, Fort Bend, Brazoria) and Austin (Williamson).
It's started to perform all other big city counties do (Cook, Los Angeles, ect.) Unless there's some major political realignment where minorities and urban voters begin to support the Republican party, the GOP doesn't really have a chance there.
There will always be a residual GOP presence in Dallas that Chicago and Los Angeles do not have. There's a lot of money in Dallas and that money is all conservative. They don't have "Lakeshore Liberals" like the Windy City and there's not a large entertainment or legal services industry like Los Angeles to provide a base of rich liberals. When Rick Santorum started that Christian movie studio in Dallas, he said part of the reason for the location was that there were a lot of wealthy evangelicals who could provide the financial backing for his movies.