It is necessary. Power may be out. Internet may be out. Your router maybe out. Alarm could be in meth head brother’s room on the other side of the house. Please build a smoke detector with dual sensor that is hardwired (battery only should not be used if you can avoid it) with interconnect support using existing wire or with wireless. But… if they can’t make one with < $40 price range, it may be a hard sell since the existing 10yr battery models with all the bells are selling for less.