You simply can't get that level of waterproofness with a draw cord closure with a lid.

Ordinary dry bags are cheaper and much simpler to make than putting shoulder straps, waist belts and assorted other attachment points on a dry bag. All those attachments to the bag just create extra opportunities for failures.

You can get the same amount of waterproofness just by using a regular dry-bag as a pack liner inside a drawstring-closure pack. You can also get perfectly adequate waterproofness just by using a contractor's trash bag as a pack liner to keep everything inside your main compartment dry, provided you don't submerse your pack. They are very cheap and work well.