I don't understand your logic, email is a way more open and universal format than PDF in terms of servers, documents (ie emails), and clients.
You have more chance of the PDF format imploding than email IMHO.
CD/DVD? Those have long (like years), been shown to degrade and are not long-term touchless archive storage media.
To be sure of retrieval though, you would have to maintain document, storage, OS and access application, as long as you make sure you always have those in a supported stack you will be good to go, whether they are proprietary or open.
TBH though in reality providing I can go back 10yrs (with the exception of photographs), I'm happy.
I think that most people would regard PDF files as a pretty much universal format, and likely to survive for a long time as a file format.
I agree that in some respects, some email formats will probably survive also since they are in widespread business use. On a personal level, however there can be issues. One is that the files are "database"-type files rather than simple, individual files, and may be more easily corrupted. After having problems with a desktop computer, I switched to a new Mac. I saved old email files by exporting to ".mbox" format. This appeared to work at first, but when I tried to import these mbox files later, a few were lost, maybe due to file corruption. All of my text files, picture files and PDF files were fine; only some of the mbox files were lost.
In addition, there can be other issues, not associated with the email format itself, but with the way we commonly use emails. For example, email accounts on my devices are set up as IMAP accounts. The inboxes on my main computer, mobile devices (iPad, iPhone) are synced with the email server. Whenever an email is deleted on any device, it is deleted on the server as well. To save an email, it should be moved to a locally-stored folder (either another "mailbox" or perhaps a PDF file). I would personally be more comfortable storing/backing up and archiving individual text or PDF files than mbox database files for future access.
That's not to say that emails cannot be successfully archived in an email file format, just saying that it can be easier (on a local, personal level) to store and automatically back up "critical" files as PDFs that can be accessed in future years on whatever device we might be using.
I'm not sure that I understood your disagreement with my comments on CD/DVD format. It appears that we agree on this. In addition to the physical degradation issue, we may not have easy access to CD or DVD drives far into the future, at least in terms of easily connecting them to whatever computers or mobile devices we will be using.
