Barrel plugs are not better technically, but it would be better to use them with random voltages because people don't assume that they conform to a standard. With barrel plugs, the customer expectation is that every device has its own charger, and that's that. With USB, the assumption is that they are all compatible.
I keep power supplies around when the device they were supposed to power goes out of service. The collection has grown to a big crate of assorted power supplies where I can hunt for a spare. It is common that I have to solder on a plug from another power supply, but it's not rare to find multiple compatible supplies that have the right plug and specs.
The scenario you describe, where I'm looking for a power supply among a set of devices, to power another device, that is less likely to turn up a winner. But chances are non-zero enough that I do wander around looking for other options when a power supply is missing.
I have found this not be true too often. Then again maybe I have more computing junk than most.
Really it should be a manufacture requirement that the barrel port on the device have a waterproof hard to destroy sticker, or case moulding that lists it's requirements. As the years have gone by I've seen way too many devices with a plug hole and no idea what it thirsts for, leading to deep searches on the net for the product. Also the wall warts themselves are commonly made by some other group, so once separated there is little to no visible pairing identification other than looking for output/input power on both devices.
I've recently made a habit of labeling power adapters with the device it belongs to. Too often I have wanted to use an old device I have not used for a few years, then had to go through my box of old power adapters trying to match up the plug, voltage, and current to the device in hand.
I mean they are multiple sizes but there are certainly standards. When they were more common I use to have boxes of them and could always find barrels to fit anything I needed and swap over the power supply to match whatever power I needed. Once in awhile you would get lucky and have both a matching plug and supply to match your need, like my electric piano uses my old Sega Genesis wall wart and plug because they matched voltage and barrel plug size and a cat chewed the original piano one and cause it to short and burn out.
