Fatal flaw? I think it's safe to say that the more time goes by, the less risk of a 'fatal flaw' there is, especially with how the developers are pretty damn good in what they do.
The risk is decreasing with time, but the risk is also non-zero and is pretty significant. The entire design is experimental and the protocol is a mere decade old. We're still finding really serious bugs in the reference implementation as of a few months ago. There's a reason Core is still in beta.
I have a lot of faith in Bitcoin and its developers, but people are right to be skeptical.