I can go put in Space Invaders (2600), Super Mario Bros, Streets of Rage, Ocarina of Time, Crazy Taxi, or Halo 1 whenever I want. Those games will always be available to play. If the Durango is online only then that means at some point, after about 8-10 years when support of the console is discontinued, every single game released for the system will be unplayable. Could you imagine if all movies or songs or books ceased to exist after a decade? That's a fucking nightmare.
That's an excellent point. What they are hoping will likely happen is that they could sell you your favorite game again on a future console.