So what. That's a little over an hour [1], and you're done! Some smallish JPG is all that's presented here anyways, so using a reasonable MP capture to JPG should easily fit on its SD card.

Also, there's around 4600 that are pure white, and something near that that are pure black, for the scene above (although more dynamic range would be very cool).

[1] 18000 * 0.5s shutter / 3600 = 2.5 hours for worst case shutter, /2 for average = 1.25 hours of exposure.

If you consider how long lower speed shutters will take and the aperture combinations, it would take a long time to take all the pictures and would stop being feasible.