You stop paying for a network you'll never run

A general-purpose BNG carries the cost of every feature it has ever needed to sell to a national carrier — in licensing, in hardware, in the support contract, and in the complexity you inherit whether you use it or not. A WISP serving ten thousand subscribers is subsidizing PPPoE session engines, ATM interworking, and protocol stacks it will never touch. Quantum IPoE BNG strips that away. A dedicated IPoE platform on commodity x86 means you buy the subscriber-management capability you actually deploy, and nothing else. The economic gap over a bolt-on BNG isn't marginal — it compounds with every subscriber you add.