Things are unveiling fast. Three years from now with another halving bitcoin would no longer be the most secure cryptocurrency against 51% attacks. This is because security is measured by network mining income for pure proof-of-work currencies, and by which measure bitcoin network is losing its share at a fast pace.
Three years from now Bitcoin will still be most secure cryptocoin, because it is most usefull coin and thus most valued. If some coin has 1/10 value of Bitcoin it is reasonable to assume the coin is 1/10 as secure