Having an XML sitemap is optional for SEO. They don't help rankings and they don't usually get pages included in them indexed. Including a new pages in your sitemap is unlikely to get Google to index them. See The Sitemap Paradox.
If you want your profile pages indexed, you need to find a way to link to them. For example you could link a profile to their friends' profiles. That way Googlebot would find them through links and would be able to judge their popularity with PageRank (link juice.)
It still could be a good idea to put your new pages into an XML sitemap. Doing so will get your new pages crawled quickly. However, without any links to them, Google is usually going to decide not to index those new pages.
Google is never going to penalize a site for removing links from your sitemap. XML sitemaps don't have enough influence over search engines for them to matter enough. Incomplete sitemaps are fine and they won't hurt your site in any way. URLs that are removed from your sitemap won't get de-indexed because they are no longer included in your sitemap. See SEO Myth: Google will only index the pages listed in your XML sitemap
Another sitemap strategy that could work for you is generating a new sitemap with only the new pages and publishing it on a new sitemap URL. If you kept all the old sitemaps, you would end up with all of your pages included, while not having to regenerate your full sitemaps. At most your would just have to generate a new sitemap index file that lists all of the sitemaps.