Googlebot is just a computer program. Computer programs like when things are simple and easy for them. Whenever possible keep things simple.
If your out of stock products are going to be replenished in a few days I would not worry about it. Even if you made a more complicated sitemap to deal with out of stock items there is low probability that it would help you. Because Google would need to immediately see your sitemap, then prioritize googlebot to recrawl your deep product pages (which probably have no backlinks) then recrunch the Google index to deal with the out of stock pages. Then when you have the product in stock Google would have to repeat the complete process. The process of adding & dropping pages to Google typically takes many days/weeks.
If you out of stock items are going to be back in a few days, it is likely your page would be fixed before googlebot even gets a chance to revisit it.
Well indexed is not very valuable to me and I'd let it die.
Good link popularity & high rankings are valuable to me. If I had a product page of a permanently expired product with 1,000 sites linking to it I would add a section to the page containing links to similar alternatives. I might even rewrite the description of the expired product to help users realize the alternatives are better choices for them.
Google uses an xml sitemap as a set of suggestions - but it doesn't have any effect on PR or rankings. Those will depend on inbound links, so as long as the links are there, dropping it from the sitemap shouldn't be a problem. Neither would leaving it.