Arrow's departure and the Broncos reluctance to get into a bidding war says more about the talented forwards coming through the ranks then anything(his not the only hardworking forward with a quality offload coming through the ranks) and I don't think Bird was exclusively signed to be lock necessarily either(not saying it's a position you could rule him out for but there was always speculation of him playing a number of positions) so I don't consider him directly at least in competition/replacement for Arrow.
Frankly with what we have seen of Bird this year it's hard to be thrilled with his recruitment considering it's hard to get any accurate reading of where he is truly at given that his entry to the club was hindered with injury, his overweight and then when he did take the field poor form, positional shuffling and injuries would be present, how he responds to this(assuming his still at the club) is open to speculation. I hope for his sake and ours that if he is here next year and beyond it's much more promising.
Arrow and Bird are the least of any concern for the the side in general however, we have more pressing concerns like getting our spine in order. Infact in the scope of things the forwards, especially the players coming through the ranks is the highpoint of our squad and our season in general.