I have only two packs, one for dayhiking and one for backpacking. The backpacking pack, a Six Moon Designs Comet, is plenty big enough for trips of 9-10 days, using hardly any of the extension collar, but I can compress it down for weekend trips. While I do scrunch it down to use as a daypack when I'm dayhiking from a base camp, I wouldn't want to use it as a regular daypack. The model has been discontinued, and I want to save it for backpacking only. Actually, a cheap book pack is fine for dayhiking--you don't have to spend a lot of money!

If you are serious about lightening your load, go to the home page of this forum and read the articles listed on the left side of the page. They will show you how to lighten your load without any danger to your survival. I was able to cut my pack weight to less than half just from these articles, especially the 7-day gear list.

As for "survival," if you take care of your gear and know how to use it, you don't need double or triple backups for everything. You do need to be prepared to stay warm and dry if the weather gets messy, but if you keep your insulation dry, as you should, you don't need multiple backups for clothing, either. Surviving bad weather and other problems is a matter of skills, especially keeping your gear dry and thermoregulating your body temperature, far more than gear! Practicing these skills in horrible weather close to your car takes a lot of will power, but it's the safest way to learn to cope!

BTW, just to avoid confusion in terms, most of us use the term "bag" to refer to a sleeping bag rather than a backpack.