I remember this one landlady complaining how her variable rate interest-only loan had rates going up so she had to increase the rent to cover it, and wasn't ent

I remember this one landlady complaining how her variable rate interest-only loan had rates going up so she had to increase the rent to cover it, and wasn't entirely possible... Like why that sort of insanity was possible in first place. There should be something going towards principle. As not doing it means that increasing rates can be destruction.