A large ruck, framed pack or any other big arse 40-70+ pound pack have their place, but they are different from a conventional bug out bag. Two different purposes.
Bug out bags got their start in military special operations circles. These dudes carry really heavy rucks to support themselves off-grid in a war zone. Radios, batteries, explosives, lots of ammo, heavy MREs to last a week or two...gets heavy fast.
They also know that if/when they make contact with the enemy, they will be unable to move quickly and make a hasty retreat with those giant rucks. So, what do they have to do? Ditch 'em. Leave them behind.
This is when the bug out bag - also called a 'go to hell' bag in some circles - comes into play. The plan has gone to hell and they need to ditch their ruck and get out of dodge. It is a smaller, lighter bag that has survival necessities. It's something they can move quickly with and even fight while wearing. And it allows the special operations guy to ditch his ruck, break contact and still have the critical essentials to either continue with his mission or make it to friendly territory.
Will they be comfortable, warm and cozy? Not likely. BUT, they will have enough to scrape through, not freeze to death or die of dehydtration.
SO - that is a 'real' bug out bag and its original intention - a smaller kit to take with you as a last ditch when retreating from danger.
At this point, we may need to come up with a different name for bug out bags in this context, since the name has been adopted for any variety of emergency backpack, big arse 80+ pounders on down. I'm certainly guilty of this. "Go to hell bag" might work.
So - a 'real' bug out bag is not something that you're going to want to take on a 5-day backpacking trip for funsies. It's not something you're going to have room to carry pots and pans and grills and three changes of clothes in. It will be small-ish, light and packed with the bare necessities.
Obviously, large rucks for supporting extended operations in the field have their place. No argument there. But, a ruck and a bug out bag are different things, at least originally - different tools for different jobs.
If you're starting out, start with a bug out bag. If you decide that you need a ruck, you'll still want to have the BOB anyways.