Actually, the attach rates tend to go up or flatline. They can fall, but thats not usually the case.
Per Nintendo's quarterly shipment info, the Wii attach rate has gone like so in the Americas:
| Wii (Shipments) | Dec-06 | Mar-07 | Jun-07 | Sep-07 | Dec-07 | Mar-08 | Jun-08 | Sep-08 | Dec-08 |
| Ams SW LTD | 9.02 | 14.49 | 23.19 | 33.49 | 60.9 | 79.37 | 98.71 | 122.25 | 169.66 |
| Ams HW LTD | 1.25 | 2.37 | 3.81 | 5.46 | 8.85 | 10.61 | 13.11 | 15.19 | 20.4 |
| SW/HW Attach | 7.22 | 6.11 | 6.09 | 6.13 | 6.88 | 7.48 | 7.53 | 8.05 | 8.32 |
In the first quarter, when only Nintendo die hards had the patience to track down when it was all but impossible to find, they bought alot of games. It became a bit easier after the holidays so the attach rate shrank to 6.11 in March flatlined essentially through June and then began to rise, without interruption since.
Dec 06 quarter had...Zelda, Excite Truck, Rayman, Madden, Red Steel, Call of Duty, do pretty well in the launch window as well as stuff like Trauma Center which did ok.
March 07 quarter had...Sonic, Tiger Woods, Wario Ware, Wii Play but the drop is understandable per wider audience appeal.
June 07 quarter is more of the same. Super Paper Mario and not a whole lot else
Sept 07 quarter had Metroid Prime 3, Madden not much else. Attach rate still pretty much flat.
Dec 07 quarter had Guitar Hero III, Galaxy, Mario & Sonic, and alot of complimentary stuff like Zack & Wiki and Fire Emblem so the attach rate really jumps here
March 08 quarter had Brawl and alot of complimentary stuff
June 08 quarter had Wii Fit, Mario Kart, more music games I believe and other small stuff
Sept 08 quarter had Madden, and alot of small to medium third party stuff
Dec 08 quarter had Wii Music, Animal Crossing, and useful compliments like De Blob, Guitar Hero, Rockband, Call of Duty, Warioland
In the case of Wii at least, with Punch Out, Wii Sports Resort, Mad World, The Conduit, Sky Crawlers, more music games, more sports games, more movie tie-ins, whatever else Nintendo has for the year, and the "new play control line" the attach rate in shipments should be over 10 by the end of the year, and with ~30-35m Wiis in the Americas by that point thats a lot of software. I dont think 320m in Wii sw shipped to the Americas is impossible by the end of 2008
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