Attach rate is the fakest stat in sales...
If game a sells at a 10% attach rate with 1mil sales, while game b sells at 5% attach rate with 2mil sales. Game b is still the better performing game. the attach rate does not necessarily hold while the total userbase increases. It almost always decreases.
So the sequel to game a is not guaranteed to sell more copies just because the total userbase of the console increases. Attach rate tells me nothing, imho