Questions should be deleted when their content no longer adds value to the site. I would be careful about preserving posts because of their view count. Users will imitate what they see, an low-quality posts cause others to leave the same.
Failing to clean out these low-value posts has been the primary reason this site has not yet graduated. It needs a lot of cleanup. You shouldn't be so hesitant to do this routine maintenance and upkeep to increase the total quality of the site.
Posts are closed for a variety of reasons, so consider these close reason to help determine whether whether they should likely lead to deletion:
Exact Duplicate:
It depends; Look at the context of how they are asked.
You'll want to keep the post if the wording provides another way for a search query to find the content. It's a bit like a "see also …" entry in an index.
Delete it if the duplicate does not add terminology or alternate phrasing to find the question. It may not be worth cluttering up the system with this exact duplicate.
Off Topic:
Almost always delete it.
Off topic usually says "This shouldn't have been posted here in the first place."
Not Constructive:
This is a judgment call.
Keep it if it says "Fair question but we discourage this so, while it's worth holding onto the information, we couldn't let it continue."
Delete it if it says "this is nothing but a 'broken window' and we should get rid of it."
Not a real question:
Almost always delete it.
Closing it before deletion provided ample time for the author (or community) to fix it up. If it wasn't re-opened by this time, it should probably go.
Too localized:
See "Off Topic".