I find it difficult to imagine a scenario in which XMR goes to zero. I find it difficult to imagine a situation in which XMR sees a dip below 0.0014 for more than a few hours (dumping case) or days (major technical issue case), for that matter. Not just now, but forever. What do you imagine that could do it, short of a full-on criminalization of the technology? Any technical issue will be fixed in short order, if not by core, then by others. Even full-on criminalization will be a short-term hurdle, when I2P integration is effective. To say it probably will go to zero seems ludicrous to me.
I don't imagine an instant collapse. More likely is a slow slide to zero as it goes nowhere, largely due to market as opposed to technical factors. This could include failure to achieve critical mass at a time when the market perceives this to no longer be plausible, leading to a downward spiral of liquidity and capitalization.
There also could be some partial instant collapses, such as the release of a coin based on zerocash or a Bitcoin ring signature sidechain.
Possible reasons:
1. Bitcoin-style cryptocurrency not seen as useful (enough) by the market. Leads to same sort of downward liquidity spiral as above, but for the entire cryptocoin universe (except maybe other platform concepts such as Ripple, fiatcoin, etc.).
2. Private cryptocurrency is not seen as useful by the market. Ditto.
3. Bitcoin is perceived private enough or becomes more private (incl. sidechain)
4. Zerocoin is perceived as trusted enough (both in terms of setup and cryptographic maturity)
5. Dark/dash is perceived as private enough (and/or is improved). Requires Dark/dash to also not collapse into a financial smoldering wreckage, or other non-privacy-related failure.
6. An unknown black swan technology replaces Bitcoin, and is perceived as private enough or has a private version.
7. An unknown black swan technology replaces Monero's ring sigs, etc., and is perceived as private enough or has a private version. Monroe does not adopt/adapt, or fails to do so quickly enough.
I'm sure there are more.
But overall, I'm fairly convinced that failure to "take off" at some point will lead to the liquidity implosion into irrelevance (maybe not literally zero for very long time, but eventually that).