Honestly, horde mode MP is fairly prevalent and the ME implementation of it wasn't anything special or original. MP design absolutely does take away from SP development, its well supported by dev comments and you can even see it in some of the map designs in ME3 specifically. I'm with Jivaro, I'd rather they focus on making a great SP game with the most content possible. I can get my MP fix elsewhere.
Better yet, just make a separate ME MP game which is really how it should be handled - $20, supported by their shitty grindfest booster packs and DLC. Totally divorced from the SP stuff. It could sink or swim on its own merits.