Plus, most people who travel with a backpack just buy a regular bag, and then spend $25 or so on a light duffel bag. When you're boarding a plane or a train and they want the straps out of the way, just whip out the duffel and stick the pack inside. As for the zip-off daypack... well, sure, but do you really want a neon sign that yells, "Tourist here!"?
Still, I can give you three choices. One is Lowe Alpine's Frontier 75 ($179; www.lowealpine.com), which is billed as a travel pack. It's a good all-around pack but does lack the stowaway straps that are the hallmark of such packs. Also, no removable daypack. It's big enough, though—75 liters with an expandable bag collar that lets you squeeze in an extra 15 liters as needed. Germany-based Deuter sells some of its packs in the U.S., including the Quantum 70+10 ($219; www.deuterusa.com). It's a travel-specific pack that will eke out 80 liters of capacity with the collar expanded. Plus, along with that all-important zip-off daypack, it also includes an integrated travel cover for airline check-in that does double duty as a weather shield.
Finally, I think you missed a pack in the Eagle Creek line—their Ultimate Explorer ($250; www.eaglecreek.com). It's a big (88 liters) pack, with a removable daypack, stowaway straps, and even a compartment for your sleeping bag or dirty laundry. Which to me sounds exactly the bag you're looking for, no? Hell, it's even a panel-loader!
Check out Outside's Buyer's Guide for the planet's burliest and most stylish travel luggage.