Q:

Are link-only answers fine if they link to Super User questions?

Let's say someone asks a question: How do I upgrade a widget for defrobulator?. There are many possible answers, each one correct and the question isn't too broad. (And defrobulator in this case is a piece of software, so it's on topic.)
One could, for example, use defrobulator gizmo. The question How to use defrobulator gizmo to upgrade widget? has already been asked, it has an accepted answer and is of reasonably good quality, so rather unlikely to be deleted.
The new question is not a duplicate, because it's broader and there's more than one solution. (One could, for example, run the bifrobulator script and upgrade defrobulator widget that way. It could be a preferred solution for some users, because bifrobulator package is available out of the box.)
What's the correct approach here?

Post an answer with a link and few words of explanation that it's one of possible methods?
Post a link-only answer and make it community wiki?
Quote that answer (or reword it, possibly making it more valuable) and flag older (more specific) question as a duplicate? (related: Should older questions be flagged as duplicates of more recent ones if those have better answers?)
Comment on the question, leaving a link to the older question?
… something else?

This question is inspired by this answer, but it's not a good example for discussing this exact issue.

A:

Link-only answers are never fine.
If you need to link to a SuperUser question or answer in order to answer a question, vote or flag to close the question as a duplicate of that question.
If linking to the other question does not provide enough information, but you want to provide the same type of answer to express that it's one possible way of doing that, you could, as you say, reword that answer. 
Remember that all contributions to SE are Creative Commons licensed, so it isn't plagiarism to copy and paste an entire answer wholesale, as long as you link to where it originally was.
And remember: if you find questions that have tons of possible answers, and other questions are restricted subsets of that answer, you may want to consider closing or flagging the question as Too Broad. I'm not saying all questions that are a superset of some other question are Too Broad, but in many cases that's how it works out.
The first sentence of this answer still applies regardless of any situation. In every single conceivable case where you might be tempted to post an "answer" on any StackExchange Question & Answer site where the entire contents of that answer is only a link (and perhaps some text that says "One possible way is here"), there is some other action that you can take which would be

Preferable
Less likely to cause your answer to be deleted and/or downvoted
More helpful to the user.

To book-end this answer once more:
Link-only answers are never fine.

A:

If you don't have a lot of time but want to be helpful:

Comment on the question, leaving a link to the older question

Otherwise, I'd go for:

Quote that answer (or reword it, possibly making it more valuable) and flag older (more specific) question as a duplicate

Generally, links to other posts on Super User are not as prone to link rot as external links. It's quite reasonable to link to another answer, without repeating much. However, this is often an indicator of the question being a duplicate (in the sense of being answered over there, not being the same question). In that case, what we'll often do when we see these answers is 1) deleting the answer and 2) closing the question as a duplicate.
In the specific case you gave us in your post here, if we'll find that the current question is more general than the older one and answers to it would be equally applicable, we can reverse the typical duplicate direction.
Taking the content from one answer, extending or rewriting it (with proper attribution) is always fine, provided that you're not just blatantly ignoring to vote/flag to close the question for your own reputation gain.

