I think you are the one not understanding at this point. This has nothing to do with "how encryption works".

Whatsapp decided to go for E2E encryption for commercial and marketing purposes following all those privacy scandals.

They did not have to. They could have gone with P2P encryption, i.e. that their databases could have stored messages in cleartext. That way authorities would not have needed to ask for any backdoor. As already mentioned, this is how cellular operators work (that's obviously something governments wanted).

I think we might see legislation brought in in the future in order to force this.