What is the concern for an influx of wild beasts? Hashem can save Yisrael from them, just like He will save them from the Cana'anim!
Rashi: Because Yisrael was less numerous than the nations that they were destroying. Consequently, until they increased, wild animals would inhabit the sparsely inhabited towns. 1
Targum Yonasan: The wild animals will come in droves to devour the corpses of the enemy that Yisrael killed.
Hadar Zekenim (20): The verse means, lest the land be desolate of Cana'anim, and when you see this, your Yetzer ha'Ra will increase and incite you. It is called Chayah - "v'Chayas ha'Sadeh Hashlemah Lach" (Iyov 5:23).
Ba'al ha'Turim: Hashem knows that in the future they will sin, and they will be Hefker to the Chayos.