Let's say on www.site.com, some products are discontinued from time to time.
Each product page has nicely laid out information for users like, product features, manuals, buyers guide etc., So, in essence it's HTML, images and PDF documents.
I see two ways of discontinuing the product,
 Remove a product page from the website and return a 404 error.
Pro: Obsolete product pages die slowly on search engines
Con: The site returns a 404 error so, users navigate to another site as they couldn't find a specific product on www.site.com
 Keep the product page as it is with a message like "this product is discontinued ... "
Pro: Useful product related information remains intact, which can be helpful for users. So, if the information is unique and is not easily available on other sites then, the product page could be helpful for users.
Con: If like, 50 products exist on the site and have the message "product is discontinued" then, it's not a pleasant experience for users.
Note: Obsolete products pages might not be deleted permanently. A good example to illustrate this is, 'seasonal products'.
If a product page has a good PageRank but, is not sold throughout the year, how to deal with it?
Any pointers to resolve this issue?