Almost all email service provider rely on domain, where domain system is centralized, so it's ridiculous to claim it's highly decentralized. However, it's different case if the email service uses onion address.
please note that the email is the ONLY neutral, free (non proprietary) and open protocol/technology for communication in the world that its infrastructure is well-established and is accessible all over the glob.
See Matrix protocol.
By this rationality even using pure TCP/IP is a centralized solution!
Please note the fact that decentralization can be interpreted in different depth of abstraction. By the way if the email bothered you, as you mentioned the wallet can use Matrix, Nym network, Onion, I2P, classic central servers,... these are all communication mean and the wallet can simply use one or more in parallel.
Although I still believe email is a perfect choice, and in future we will see the wallet users will choose which solution.