I think it would have been considerably bigger than it was the one that eventually came out. The feeling of the record, that excitement and fevor that comes alongside side a great record, that's what people were expecting from U2, not songs that sounded like U2. I don't think anybody was expecting How To... to sound the way it ended up (and not in an awesome Acthung way, but rather "sounds like warm milk and in bed before 7").
Native Son wouldn't have been as big a single but would have been a bigger album seller as even super indie dudes who say even Animal Collective are unorginal love that song, considerably over Vertigo, which sounds like an album opener, but thematically doesn't really make any sense at all for it.
It also would have prefaced the Killers, Arcade Fire, etc., and since the album has a similar Joy Division-like sound it would flatlined even more people as of being something new, plus the return of Joy Division music, which everybody loved the prior two bands for. Younger people like myself would have gone nuts over hearing Native Son, then All Because of You (alternate), then City of Blinding Lights, then Mercy, then Love and Peace or Else, Sometimes..., as singles. It would have been huge.
The timing for it would have been aweseome considering it would have come out just before 2004: the American elections, Iraq exploding, Africa getting a chance to turn around, during which they would have toured, becoming even more relevant. In fact, I'd say they would have been the single most relevant, eyes-on-you band, as opposed to this time around when they were simply a really big band touring.
Bono's message of equality and dreaming for a better world have benefitted hugely from all this as well. What album released in 2003 would have competed with this one in terms of total wide-sprung I-need-this-ness?