Why are you assuming they would cannibalize? There's nothing stopping people from buying multiple games in the same genre. I doubt people go "ok this year I'm going to buy 1 platformer, 1 puzzler, 1 shooter." Plus the reverse can be said too, that more games of a genre means more fans of that genre will be made/buy the system therefore increasing the sales of games in said genre.
If there's a small enough release gap separating games of similar genres, it could be the difference to a lot of potential buyers between buying all of them at full-price or buying one or two at full-price while leaving the other ones for 2nd-hand purchases, sales or irrelevance. I know I remember doing that a lot when I was a teenager with a limited income.