First off, posts that contain links and no context are not good in the first place. They are susceptible to linkrot, unknown edits, and they simply are not an answer.
So the best practice is, as you mentioned, to link the original and:
Add a quote if the context is needed for the question;
If you look at the preceding paragraphs, they would all make perfect sense even if the links were removed.
Resolution in the current system
10k users only is accepted practice for links where the issue is specific to a page that most users can't see.
- Do Not remove the link. Questions get deleted and undeleted. So the link is/was relevant and should stay as it is, unless the author would like to replace it with a different link that highlights their point.
- Injecting a contextual quote is best practices in all cases. Do this even if the link is valid.
Solutions That Require Implementation
- Don't delete posts that are already referred to;
- Warn administrators when a post gets deleted that is referred to;
- Make it impossible for mere mortals to do delete questions that have been referred to;
First off, deletion is a very important tool for keeping the site clean. There are rules in place to make sure that deletions are not arbitrary which means that there's usually a strong reason for deleting it. Stopping that because someone else linked to it is unwise. It would also create an enormous loophole for the users who don't want their questions deleted. That takes care of 1 and 3.
For item 2, I don't think the administrators really need to care. Which is why I've created this:
Suggested Resolution (with pseudo implementation suggestion)
Every post (and comment) would need an extra DB column called "IntraLinksTo", and which would be a list of PostIDs.
Every time a post is deleted, a simply query would be able to determine all the posts which link to it, and the link could be followed by
"(10k users only)"
It could also be reversed if a post is undeleted by querying and then deleting the added text.
This has the advantage of simplicity and automation. It doesn't generate or modify any content in a meaningful way, simply adds one bit of meta data if necessary.