I completely agree. It's about time Paris-Roubaix adapted to modern cycling and increased its mileage.
San Remo, Liège, and Lombardy are fine. This year San Remo was a bit longer, and Flanders is tough enough with its distance. Even so, I wouldn't mind adding a few more kilometers to Flanders, Liège, and Lombardy. Monuments are agonizing races, and Paris-Roubaix is too short for the current ciclying.
I would also increase the mileage of the mountain stages in the GT races, as they used to be, but that's not a topic for this thread.