Still seems to have plenty of vulnerabilities within a codebase that will never be updated anymore.
I don't get this obsession with updates. If it's secure now for certain use cases then an update could only put that use case at risk. It's not like it can get "more secure" or something.
Can you clarify what you mean by this? What sort of vulnerability could magically appear in Truecrypt to make it less secure now than a decade ago?
That said considering the circumstances in which the "developer" of this software disappeared, how it was presented and the fact that it is not maintained anymore should make people think twice about using it.
An attacker knowing these flaws now has a smaller attack surface than a decade ago, since he now knows some weaknesses.
There's the magic.
First, nothing is ever totally secure. If a system gets audited today, the best the auditors can say is that to the best of their knowledge, the system has either no flaws, or list the vulnerabilities they know about. There might be vulnerabilities they missed. Some of the unknown ones might be blatant (say, a backdoor), or very subtle. When the unknown ones are found later on, and become public, the only way to not be vulnerable to those is to update.
Second, a system may become vulnerable later by the environment around it changing. This might be, for example, a change in the compiler (you rebuild for a new platform, and the kernel introduces a vulnerability), or in the language interpreter, or some library that the software uses, or the operating system kernel, or something else. It might be that you upgrade the CPU and the hardware random number generater on the new CPU is worse than in the old CPU. It might be that you move your system from physical hardware you control to a virtual machine you rent, thereby violating security assumptions made by the software.
A system, or software, that is never updated stays still, and never gets better, while potential attackers learn more tricks and more ways to attack. Sooner or later they'll find a way to attack any stale systems.
And that is why updating is important for security.
If you use a software which isn't maintained, it's insecure not because of its vulnerabilities, but because of its environment, which is ready to attack the vulnerabilities. From a general point of view, it's not risky to use a software from 2005. It's just risky to use it in 2015.
Try installing a Flash version from 2005 and open a web browser with it in 2015. That's basically killing your computer, and yet we were all using this Flash version in 2005 without a problem (or almost).
What would be nice would be for these researchers to publish those fixes. And let the unofficial forks (like Veracrypt) to incorporate them.
Bruce Schneier for example is some one who's able to design actual ciphers and cryptographic systems, but I'm not sure if he can actually "build" them as far as software systems go. From what I know of him, his works, and from reading his books he's a traditional computer scientists and much less of a developer, and I'm really not sure if he's up to date with the current C++ language standards and compiler architectures or insert what ever language you want here, in fact I would bet my money on the fact that he isn't but he's more than capable enough to find the right people to do that work for him.
That said based on reading the audit report form the TC audit the findings are detailed enough so if you are capable of truly understanding them you should be capable of finding a way to resolve them, if not you shouldn't be developing cryptogrphic software in the first place.