I've felt for a while that the main thing to ask yourself wrt crypto speculation is when does a pessimistic narrative dominate an optimistic narrative. I've argued that crypto, being software based, is inherently optimistic since any present day deficiencies can arguably fixed with "more code." This doesn't have to be true it just needs to be convincing enough to generate enough demand for the next wave of speculators.
In the limit, either the concept is fundamentally flawed or it will inductively work after many iterations and rollovers to new models/coins/etc. But if it's flawed that could be a long time before the market collectively throws its hands up and says "there is no hope for this ever working" and drives the price to literally zero -- I mean, what would the catalyst be? In short term periods, market momentum can drive sentiment, but in longer periods there needs to be some kind of pure capitulation event that causes the market to resolve that crypto is doomed. Your guess is as good as mine as to what that is -- the long term trend seems to be that the last generation of holders cash out, causing a price drop, and new optimists pile in, driving the price higher than previous highs, sometimes in the form of new systems like ether or ICOs. It's hard to imagine what will break this cycle. Maybe the crypto engineering community tries a bunch of stuff and "these X problems cannot be solved in less than Y" years enters the crypto engineering zeiguist?