The benefits of doing Lordaeron first are endless and no matter whether you'd have kept the rest of BFA the same or changed it to be an actual faction war expansion it'd have serviced the plot. It'd give the Alliance the actual initiative and a starting win, it'd make Sylvanas focusing on Kalimdor make more sense as she'd have already lost her place of power as well as give the Horde an extra reason to stick by her. On top of that, while Teldrassil is only remembered because of how it got cooked, Lordaeron is a core place of the franchise since WC2 and the Forsaken-Living conflict over it has been built up since Vanilla.