It would be brilliant. Currently the Paris-Milan train line is barely competitive with flying between the two; knocking off 2-3 hours from the trip would make it around 4 hours in total, which is very competitive with flying (1h30 flight, but both CDG and Malpensa are big airports far outside the city, with significant time wasted getting to them, through security, etc). And of course it would be massive for Lyon - Turin, and Lyon - Milan too, where flying wouldn't even make sense any more.