I installed Electrum and created a wallet there. All I know is the seed generated. I don't know private keys from the Bitcoin addresses it manages for me. And I don't see an option to get them from there. So, the question is how safe this system is? Won't I lose my bitcoins one day?

I mean, why Bitcoin nodes keep all the data of all blocks, back to the genesis block? The majority of this information is not required for processing new blocks and may be ignored. Well, some statistical resources should still keep it, but to run a node it's possible to just delete the majority of historical data. Am I right?