Please suggest me some way to recover my wallet if Electrum is eventually not available in the future.
Electrum is a deterministic (HD) wallet, and has been for several years now. When you create a new wallet, you are provided a 12-word seed. Write it down and keep it safe. If you ever need to, you can recover the wallet from the seed. An Electrum seed won't be compatible with all wallets, but you should be able to derive your keypairs using
Ian Coleman's open source tool.
Even if they stop developing Electrum, old versions will still exist, so you can import your wallet and dump your keys.
More on HD wallets here:
https://en.bitcoin.it/wiki/Deterministic_walletImagine what would you do if you had a million USD in your wallet...
Do you think it would be a good idea to store the public and private keys into some USB drive (or print them with their QR codes) and store it somewhere safe?
Yes, it's prudent to make multiple back-ups. I use several mediums as well:
-Encrypted wallet, backed up on PC, USB or other media
-Paper wallet
-HD wallet seed