The concept of email is too fuzzy. Is it SMTP that will last forever? This technology is more like sending postcards that every middleman can read. Certainly a more private alternative will replace it someday, and it should be more spam-proof.
Also email serves many purposes. If it is to be replaced, it will be on a use-case by use-case manner. RSS is a better alternative to one-to-many mailing lists. Forums are a better alternative to many-to-many mailing lists. Etc.
But yes, we should improve what we have until we can replace it.
Encrypted SMTP is already in widespread use. Many MTA's already receive and deliver email over secure channels, and email clients (e.g. Outlook) have supported secure channels for a long time.
While SMTP and POP protocols can be transmitted over TLS between hosts, which prevents reading messages along the wire, the email data itself is _not_ encrypted. A compromised SMTP server can easily read and copy any message received or transmitted.
And, even if you use a side-channel to distribute keys between the sender and receiver (to encrypt the data safely), the header absolutely has to be plain-text. Governments and companies scraping email meta-data is already a huge problem.
A protocol closer to Tor would make for much more secure email distribution, but it would also require a complete protocol rewrite. Potential death of email from a back-end point of view at least.
Also email serves many purposes. If it is to be replaced, it will be on a use-case by use-case manner. RSS is a better alternative to one-to-many mailing lists. Forums are a better alternative to many-to-many mailing lists. Etc.
But yes, we should improve what we have until we can replace it.