Don’t you think they will be in their best behavior in that period because if he/she badly want to marry the other person and once they are legally wed they start showing their true colors because they are committed? I don’t think living together for a certain period gives the clear picture of a persons character/ habits. If we are in love we will know in and out of the person even if we are not literally living with him.  

 

My cousin in US (born & raised there) lived with her ex husband for an year or so. But their marriage did not last for one year. So I don’t think living together will end up in happy marriage in all cases. But as you said a couple can live happily without marriage for years if both of them are committed to the relation even if they don’t believe in the institution of marriage.