Who: By Neil de Grasse Tyson, as I recall. Not that it matters: We are in a period of exponential expansion with no apparent or near-term upper limit. This is just as much as Bang as the one 14 GY ago, and just as Big.By whom?
We do currently have a reason to believe that the CC is positive. Otherwise we couldn't explain why we observe that the universe expands accelerated.
cc: What part of "no a priori reason that the cc ... should be positive", and "[no] reason ... it could not have been negative" wasn't clear? Our current understanding of physics does not predict a necessarily positive value, or any particular value. We believe it's positive solely because the observations say it's positive, not because of GR. Unless you are aware of theoretical analysis that is broadly accepted in a cosmological context that I am unaware of...? (Always possible.)