Exactly, it depends largely on the tank design. Yes, you can make a big tank that doesn't need a center brace. You have to take the lack of brace into account when designing the frame and choosing the glass. The problem is that a 55 like this is designed to have a center brace. That fact that another tank doesn't need one is immaterial, this one needs one.
If the break had happened towards the center it would have been an easy fix. Just use a piece of plastic or metal and screw it to the brace on either side of the break. It's not pretty but it'd work just fine. That close to the glass though there's just not much you can do.
The ultimate questions are:
1) Are you willing to have 55 gallons of water spill all over your floors if you're wrong.
2) Are you willing to have everything in the tank die if you're wrong and it breaks.
If the answer to either is no then its time to replace the tank. That's not really directed at the OP, just in general when considering whether or not to repair/replace your tank or just live with damage like broken supports, cracked or chipped glass, etc.