Does it all depend on the availability of nodes? What if say there's 1 node left in Ethereum, will that node contains the entire blockchain?
Each node connected in the blockchain contains a copy of the entire chain in which consensus is held. With Ethereum, these nodes are called validators. The less validators there are, the less copies of the blockchain there are and less validators keeps the consensus. If only one validator exists, this validator controls the entire chain, which obviously leads to centrilization and would defeat the whole purpose of the blockchain.
As with how permanently data is stored on the blockchain, this then leads to how 'permanent' this validator stays on the chain. If this validator decides to 'disconnect' all data will not neccesarily be lost as it could be backed up or stored somewhere else. The only thing which will then occur is the chain will not add any more data.