There’s another advantage Plasma Debit has over a single hub payment channel construction — in short, smaller lockup requirement for the operator:
In a payment channel hub, the total value available for all of the users to transact with can never exceed the amount of liquidity the operator has provided; this is a hefty lockup requirement.
In the extreme case for Plasma Debit, the operator can provide this same 1:1 liquidity, but they can also potentially still facilitate fungible payments between users with a smaller capital lockup; this is possible to the extent that can users make payments via full coin transfers as opposed to debit-channel payments.
Some simple optimizations can encourage the likelihood of such payments being possible. For example, we can “encourage” debit coins to be deposited in base-two denominations, i.e., a single 16 ETH coin requires a full 16 ETH of operator collateral for full fungibility, whereas if we instead have coins of values 8, 4, 2, 1, 1, this requires only 1 ETH of operator collateral (against one of the 1 ETH coins) for full fungibility (this assumes we can combine transfers of multiple coins and debit payments atomically).
As coins start moving around, if a user requires fungibility that they don’t have available, the operator, who has a global view of the network, can find opportunities to ask users for parity atomic swaps (trade a 4 ETH coin for 4 1 ETH coins, say) to help out. A more interesting (but trickier) functionality would be “involuntary parity atomic swaps”, i.e., same as above, but without either user’s permission. Not sure if this has been discussed (though involuntary defragmentation is described here: Plasma Cash defragmentation, take 2); but I can’t figure out a simple way of doing this without reintroducing mass exit vulnerabilities (basically, at any point your coin can suddenly be in-flight, so you can’t tell if some arbitrary is actually rightfully yours).