Proposal: When sitemap.xml entries cross 50K, the sitemapindex.xml methodology should automatically be used such that sitemap_1.xml, sitemap_2.xml etc are automatically produced with no more than 50K URLs each, and pointed to by the parent manifest
Underlying issue: Unable to submit a monolingual site with 56K pages (and growing!) to Google Webmaster Tools indexer page: “Too Many URLs - Your Sitemap contains too many URLs. Please create multiple Sitemaps with up to 50000 URLs each and submit all Sitemaps.”
Often requested, but technically not possible. The nearest you will get is probably @ju52’s solution in the answers here:
The sitemap you are referring to is by the way more or less a sample for “normal” sites. Having 50k+ pages is a bit out of normal and you are better off doing your layouts without using internal layouts.
Thinking deeper about this, it’s probably possible to create that using a custom post type, then using the list template of that post type to count the items and link the sitemap files with x items each as sitemap index, then using the single template to create a sitemap for x items starting at filenumber*x and showing x items.