The bottom line: If your Galaxy S21 Ultra is in good shape, hold onto it for a little while longer.
Since Samsung supports four generations of Android software updates on the Galaxy S21 lineup, you'll also continue to get new versions of Android through Android 15. And if you really want the S Pen, you can always buy it separately instead of upgrading your entire phone. The Galaxy S23 Ultra's 200-megapixel camera is only truly noticeable when you zoom in at the pixel level, or if you intend to print large copies of your photos. Even then, I'd encourage you to hold onto your S21 Ultra for as long as possible to justify the money you spent on it in the first place. But these changes aren't significant enough to justify springing for a new device just yet, unless you can basically get it for free through a trade-in deal.
