Which blockchain could become just as decentralized as Bitcoin?
None. The closest Blockchain to Bitcoin was Ethereum but their recent change of protocol from PoW to PoS makes them more centralized since the network can controlled by top holders of Ethereum that includes the creator and the early investors itself while Bitcoin network is controlled by Miners were there’s no real majority hashrate holder.
Where could I store my assets in an emergency if there are no more bitcoins?
Hardware wallet! Eg. Trezor, Ledger etc..