I like Lacewing for the fact that it isn't another broken Legendary scout mech, Lacewing's power and utility mainly depends on the map. It looks like it's more tactically based.

Why replacement? Lancer is for the early game; Tengu is for the mid-game, according to their Grades. Both of them are out of place competing with Legendary Mechs, so it's a perfect time for the new Mech to show up!