Eh if they can keep up in T4, that would mean the very powerful Frozen Shadoweave is being kept up with which is what Shadow Priests keep going well into T5. Both T4 and T5 bonuses allow a Smite Priest to continue scaling.
I’m not arguing the spec should be played, but the damage it would deal with the 100% SP scaling of Holy Fire, non-stop simple Smite spam, and SW:Pain upkeep is nothing to sneeze at. It just doesn’t provide an iota of utility.