I've always had the mindset that it was worth making some tradeoffs to keep my 72 hour kit so that it fits within a daypack. I make some tradeoffs on the food side in particular, using lifeboat rations, like Mainstays, to minimize space/weight, while providing calories and nutritional content. I'd rather take up more space/weight with water than food anyway, since it's far more important.

There are a few reasons to stick with a backpack, I think:
- It allows you to have your kit with you, travelling with hands free. That's important for potentially walking/climbing over rough terrain, through damaged areas, etc. I think assuming that you'll be able to drag a large wheeled bag around might not be the best scenario to put yourself into.
- The backpack will also stay with you. If it's on your back, and not too heavy, you won't tend to take it off. In a survival situation, you want to have everything "on you" if you can. Situations change quickly, and you don't want to make the "should I go back to get my gear that I left over there for just a few minutes" when the levy breaks, or the building next to you starts to fall.
- The backpack may be easier to manage if you're injured. If you hurt one of your legs, or an ankle, you can probably brace it, and walk with an impovised cane. If you have the backpack, that still will work, if you're trying to drag something, it's going to be a lot harder.

Another suggestion, get a pack with a good, padded weight bearing waist belt. If you can keep the weight on your hips, rather than the shoulders, you can handle that load a lot easier. Some daypacks have flimsy waist straps, avoid them. It's better to spend a few bucks more for the comfort.