Honestly, how much would it break the game if ALL spells could be considered rituals? I mean, it makes casters much more versatile and useful outside of having limited slots. Besides, in pressure (combat, etc.) you couldn't use a ritual since it takes so long, so what really is the downside?
Well first, why would you play anything else, ever?
Second, and I don't even have to detail these, they're that bad; At-will Wish. At-will Simulacrum. At-will Animate Dead. At-will Create Dead. At-will Foresight. At-will 6th level spells. At-will 7th level spells. At-will 8th level spells. At-will 9th level spells. (At-will Meteor Swarm.) At-will Conjure X spells. Etc.