So I have two points for you:
> Take a look at so-called 'professional' cryptographers and their libraries, and you'll find that practically all of their code had severe bugs. Professionals make errors like everyone else, 'professional' just means you're being paid for it. You will have a hard time finding a professional and widely used crypto library that wasn't essentially compromised in one of its functions at one time or another. Not only that, professionals from the closed-source department have a long history of coming up with compromised or bogus, sometimes even ridiculous cryptographic algorithms and implementations. Two typical examples: CMEA cell phone encryption, and the Crypto AG backdoor debacle.
This doesn't diminish the claim that the modal individual should not attempt to "roll their own crypto." You're pointing out that professionals make mistakes, but of course they do - that indicates nothing about the propensity for amateurs to make mistakes.
> It requires about the same level of skill as e.g. implementing a production-ready low-level network protocol or a file system.
I have never implemented a low level network protocol or a file system (although at least the former sounds like a fun project) - so I cannot claim you're wrong here. However, I will point out that a network protocol is not designed to be error-proof in an intentionally antagonistic environment, which is a difficulty unique to cryptographic software. There are incentives involved in breaking crypto code that do not present themselves in other areas of software development, even if the programming difficulty itself is not entirely dissimilar.
> Last but not least, many popular and widely used crypto libraries started as a side hobby. For example, libtomcrypt started that way. In fact, many widely respected cryptographers started as hobbyists. For example, Bruce Schneier. Of course, the ones who are widely accepted as peers are also those who are in the 'hire me as a consultant, not the other guy' business, and they will naturally advise everyone to not roll their own crypto but to rely on their expertise...
They can start as side hobbies, but they are not going to remain that way for any meaningful amount of time if they are to be widely deployed and safe. You can only choose two out of those three.
More importantly, there is no cabal of cryptographers spreading FUD to line their own pockets with consulting fees. The cryptography consulting industry is very small compared to the actual security consulting industry. In the application security consulting industry I do believe there are firms that try to secure business this way, but that industry is much larger (and has firms which try to misrepresent cryptographic competency). I have had significant interactions with engineers at Riscure and NCC Crypto, which is a sizable portion of all the "real" cryptanalytic consulting work that occurs (at least in the United States) - they never struck me as being the sort to suggest what you're saying. That leaves Rambus/CR and a select few other firms doing real crypto consulting, which leads me to believe the industry is, on the whole, very legitimate.