And if you play 4 seamers and a spinner they might all struggle to take wickets.
And I'm still not totally sure, but aren't some of the WC stadia new grounds? In which case we don't know what they'll play like.
Playing 5 seamers might be predictable but if those 5 seamers were, say, Pollock, McGrath, Ntini, a fully fit Shoaib Akhtar and Mohd Asif, would you really worry about predictability?
'Cos I sure wouldn't.
If your seamers are better bowlers than your spinners (relative to the conditions, obviously), then you play an all-seam attack. People make too much of the "variation" thing. What matters is skill, not variation. West Indies proved that in 1977, and continued to do so for the next 9 years, and most people are still struggling to catch-up.