The fact that water drained at all is a good thing. Do you have soft water? Otherwise, if there's a lot of mineral buildup, it can act like an insulator, and fool the thermostat, or, worst case, decrease the overall volume, sometimes, considerably.
One way you can 'fool' the system to appear larger is to raise the storage temperature, but in that case, you'd also want to install a tempering valve to limit how hot the outlet could get for safety. But, because for each gallon of hot water, you'd then be mixing it at the outlet with some cold to cool it down to a safe level, it would appear to be more hot water at that lower temperature available before you'd notice it cooling off.
While dip tubes can fail, that was mostly a problem that showed up years ago, and those installed after that problem started to show up were better, and usually last. IT's not hard to check, but any time you mess with any connection on an older WH, you risk further problems. If it's more than a few years old, it probably has used up all of its anode rod as well. Those don't last forever, but if you did change it out regularly, you could theoretically extend the life of the thing. They're called sacrificial anodes for a reason...they give up their substance/life, to try to save the tank. Once they're gone, the tank can be attacked more easily. They're not perfect, but they can help. They use them on boats, bridge supports, and bouies all of the time to extend their lives.