The tank is fine with the 1/2" thick glass, if it indeed was going to fail it would of done it a long time ago. As I referenced in a earlier post, in the 1970's is when they started making tanks without the metal frames, and during the 70's & early 80's they used 1/2" thick glass for tanks 55 gallons and larger.
The tanks were manufactured without a center brace because the 1/2" thick glass is able to withstand the water pressure without bowing, as long as the tanks built in that time period were built long and not high there was no need for a brace.
If you're still worried about the tank bowing out, and don't trust it without a center brace, you can always install metal braces made out of sheet metal to solve the issue.
15g Fluval Flex - Will be a planted shrimp tank