Could SE improve that by generating a sitemap that de-emphasizes closed questions? IIRC Google gives the option to control the relative weighting of various pages without deindexing them entirely. (Although maybe a sitemap on the scale of SO would be problematic in some way? I've certainly never tried to create one that large.)
There's no size problem. Each sitemap file can contain up to 50,000 urls. Google's cache on SO's sitemap.xml file shows 126 files (maxing at 6.3 million urls (50k * 126); their site claims 6.33 million questions at the moment, so it all appears to roughly add up).
I've run sites with millions of urls spread across a lot of sitemaps, and Google will devour them without any fundamental problems. I found it wasn't uncommon for Google to regularly screwup the indexing of a few sitemap files out of a hundred or whatever, and often it would randomly fail to update a % of sitemaps in a timely fashion.
