YC funded their AJAX calendar app (Kiko) in 2005, before Google Calender or JQuery came out. Indeed, in the blog post where John Resig announced that he was working on JQuery, the first comment is by the founder of Kiko (who later went on to found and sell Twitch):
DropBox was also "hard tech" when it came out in 2007; it was file storage, but it was file storage that worked by reverse engineering the operating system so it could hook into Windows Explorer, Finder, and whatever Linux file manager you were using.
I think you're arguing away from what the YC Partners would admit. They didn't fund DropBox because Drew showed up and said, "I don't know if file syncing will work but..." He was a designer who said, "I can make the user experience easier." Whether there were aspects during implementation that were tricky is not surprising but also moot. (And remember Gmail was introduced in 2004; by 2005, AJAX was not as crazy new as you're trying to make it sound.)
YC doesn't fund "hard tech" because they want to make money and the fastest way to make money is to find new uses of existing tech. Now we've found we've driven that car as far as it would go and because we didn't invest in gas (new tech), we're stuck.
What do you mean "reverse engineering"? Linux and Darwin/Mach are open source, and writing NT filter drivers is extremely well documented (especially for anything MSDN documentation) [1].
Either way, I doubt Dropbox even touches the OS kernel. You can implement all of Dropbox's functionality in userspace using things like Shell32 and file CRUD operations over system calls.
You can't possibly tell me I need to reverse engineer my OS to write a program that downloads and uploads files to the Interwebs? Maybe inotify is the one mystical ingredient...
2007 wasn't exactly the dark ages of computing technology that you make it sound like. BitTorrent was solving the "hard problem" equivalent of Dropbox in 2001, AND it was decentralized :o
http://ejohn.org/blog/selectors-in-javascript/
DropBox was also "hard tech" when it came out in 2007; it was file storage, but it was file storage that worked by reverse engineering the operating system so it could hook into Windows Explorer, Finder, and whatever Linux file manager you were using.