Here are a few problems that I don't think are being addressed here:
1) Wallace is approaching this issue not via Scripture, but via his beliefs. We all bring beliefs to Scriptural interpretation (I'll get to that), thus quoting Scripture rarely does any good because we rarely approach Scripture so openly as to let it question our beliefs.
2) Wallace isn't necessarily approaching conversion, though the question he asks is one that many potential converts (and eventual converts) begin to ask. Rather, he may just seek a new way to establish ecclesiology within the Protestant church
3) Many SBC professors would disagree with him. They take εκκλησια (ekklesia: Church) to refer mostly to the local church. In other words, their belief paints their interpretation; it's hard to get the to move beyond this. Almost every scripture you point out to them they'll say, "That's in reference to the local church," completely ignoring how the early Church interpreted the passage.