I don't think you've seen everything Dallas and Dart are planning for downtown Dallas.
The number UNO idea in all their planning is to reduce the construction costs as much as possible, without sacrificing too much on peak hour capacity.
This of course means limiting the amount of tunneling. When Dart evaluation charts rank subways stations undesirable because the have security and safety issues, and commercially because of their low visibility; it is very apparent Dart doesn't want many miles of subway lines.
Dart light rail trains can transfer to commuter trains at either Union or Victory Stations. All Dart light rail trains will be able to transfer at the West Side station and with their sister paired line for almost half their lengths.
In Dart's phase III plans, Dart plans an Orange line branch along Scyene Road to Masters (Mesquite city limits), extending the Red line south to Red Bird (Duncanville city limits, and a new (?color?) line west towards Grand Prairie from downtown Dallas. How they plan to go over or under an expanded Stemmons Freeway (I-35E Pegasus Project) hasn't been looked at.
In downtown Dallas, Dallas and Dart are looking at building streetcar lines. These streetcar lines will be able to transfer to Dart light rail trains at Dart light rail stations. That's how Dallas and Dart plan to achieve your proposed grid pattern in downtown Dallas. Most likely, there will be at least one streetcar line in both north-south and east-west directions in additional to Dart's planned two light rail corridors.
Therefore, when you add the streetcar lines to the two light rail corridors through downtown Dallas, they don't need an extended subway line.