A restricted diet is not a pleasant thing.
I don’t think anyone would choose to eat one thing only; a definition of human-ness might include delighting in the experience of new things. We pity those who hide away from new sights, sounds, tastes, touches or smells. Art, music, haute cuisine, textiles, perfumes. Film, radio, kisses, caresses, roses.
So if I had to eat only one type of food I would hope it was something I don’t really care about. Because If I had to eat only a thing I enjoyed it would turn to ashes in my mouth after time and there would be no happy memories of that thing to sustain my soul. We eat because we must, We taste because we can. We savour because we are human.
That is all.