The FourPanesViewer is pure VTK. It has no GUI widgets, no proper DICOM parser, no proper widgets, etc. The Slicer core will not be significantly smaller than 300MB just by removing DLLs, but you would need to reengineer the application. Since 300MB is smaller than a typical image that the user downloads by the dozens and you don’t actually need to deploy the 300MB Slicer, only a 10MB launcher, the effort for reengineering (and maintain the ultra-lean Slicer core) is very hard to justify.
Yes, AI is a very trendy topic, so there is lots of activity around it in the Slicer community, too. This is a good example of why creating an ultra-lean Slicer core would not worth the effort: users would keep asking for enabling features and we would end up using the full Slicer core quite quickly.