This started out as a question, but the more I played with it, the more I began to find/understand. So, I'll just explain what was happening and what I found, fwiw.
Scenario: One of our drafters has a drawing that informs me "1704 selected. Do you really want to do this?". This is happening on the Trim, Extend, Fillet, and Grip-Stretch commands. I've seen this prompt with Hatch when using the "pick points" to find a boundary, but never on these commands before.
What's also interesting is that drawing has only 234 entites (when I try an "ERASE All"). One of the objects is an attached raster image. When I detach it, (resulting in 233 entities), the warning still says "1704 selected."
With the Hatch, you can eliminate this warning by picking objects or limiting what AutoCAD has to "think about" (frz layers, zoom in,etc.). Zooming did nothing, but freezing layers helped.
After snooping around in the drawing, I found some blocks. If I erased them, and tried fillet/stretch/trim/extend, there was no warning. Also, if I exploded the blocks, it resulted in 1817 entities in the drawing! Go figure!
Oh, well. At least I was able to fillet the lines.