I would be hesitant to draw far-fetched conclusions from this. Often an idea persistently fails until someone finally does them right and then it doesn't fail anymore. One example is document synchronization; people justified the consistent failure of synchronization solutions with all sorts of explanations (including the "people just don't need it" joker card) until Dropbox came along, did it right, and now synchronization suddenly doesn't seem to be a "dead on arrival" idea. There are plenty of other examples.
It's easy to justify the failure of a product or solution by blaming the circumstances, like the "market not being ready" or "people don't need it" or "the market is too small" etc. Often, it's just that the product sucks, which might or might not be the fault of the creators.
TL;DR: Ideas only keep failing until someone does them right.
"Often an idea persistently fails until someone finally does them right and then it doesn't fail anymore."
But I very strongly think that you should know that you are walking into a minefield, rather than just blundering into it. You should know that you need to learn the landscape, try to figure out what failed, and sail your project through what may be a very narrow window hard to find by chance, as opposed to other startup core ideas where success is more about perseverance and market savvy and a lot of other things other than sailing through a very small technical window.
Also, document synchronization is a word I'd save for actually trying to synchronize documents, change tracking and merging, etc. Dropbox isn't the first to solve the problem of presenting your backed up files as a folder, a much simpler problem, they're just the first to productize it for consumers successfully. Business and open-source-technical solutions were around for a while.
Dropbox was as much about the market being right as good execution. If 90% of people only use one device, there's little need for synchronization. Dropbox did sync right, but also coincidentally at the same time that netbooks and smartphones were making sync a valuable mass-market product.
It's easy to justify the failure of a product or solution by blaming the circumstances, like the "market not being ready" or "people don't need it" or "the market is too small" etc. Often, it's just that the product sucks, which might or might not be the fault of the creators.
TL;DR: Ideas only keep failing until someone does them right.