I think inactivity is the biggest risk at all at the moment.
I do believe most of the bitcoin value is built on a promise that bitcoin will be a viable payments technology - to get there we still need more adoption and more growth in transaction volume. As compared to other crypto currencies, the most significant value with bitcoin is the network effect - to keep staying there we also need growth in adoption and transaction volume. We are currently hitting the ceiling, we are at max throughput - all due to an artificial block size limit that was "temporarily" added to the system in 2009 or 2010 (when average block size was few kilobytes) to avoid a rogue miner to create extremely big blocks. There is no room for adoption or growth!