TL;DR
The question presents an association fallacy that correlates bit-rot with deleted questions.
- Deleted questions break inbound links.
- Bit-rot is bad.
- Therefore, all deletions of questions are bad.
Defining "Deleted"
One potential problem here is that there also exist posts that really should be expunged. This may include posts with sensitive data such as passwords, or proofs of Godwin's Law. The current system clearly differentiates between closed questions and deleted questions; are we really proposing a new system that also needs to differentiate between just sort-of deleted questions and really, really deleted questions?
Responsibility for Link Rot
Preventing link rot is one of the reasons why link-only answers are frowned upon. Answers should include actual informationshould include actual information so that the loss of a supporting link doesn't preclude the transfer of information.
Likewise, Stack Overflow is not responsible for outside sites that use links to SO as answers rather than as a bibliography. Maintaining links is the responsibility of the author or webmaster, and if links are being used for anything other than supporting information, "You Are Doing It Wrong.™"
Conclusion
Inbound bit-rot is not SO's responsibility. It will not speed up universal entropy in any appreciable way, bring on a new Dark Age, or trigger the heat-death of the universe in our lifetime.
Leave well enough alone.
