The uridine modification was intended to reduce immunogenicity of mRNA - some of our immune cells have pattern-seeking receptors in the TLR family that recognize ssRNA and dsRNA. The presence of modified uridines throws this pattern recognition off. (https://doi.org/10.1016/j.jconrel.2015.08.051)

The modifications to increase mRNA half-life concerned mostly the caps and poly(A) tail. But even with those the persistence was in the range of days (sort of depending on how sensitive a method you picked).