The EU did a lot of analysis about this between member states.

It concluded that markets with 4 or more providers were far better served (price, quality, coverage, etc) than those with 3 (which effectively is what the US will have now).

The UK blocked the merger of 3 and O2 on these grounds, as it would leave only 3 providers.

Yeah, this merger (for whatever reason) reminds me a lot of the US Air/American Airlines merger of a few years ago. On the one hand, going from 4 to 3 'major' players seems an obvious loss for consumers. On the other hand, unless these two combine, I don't really see either of them able to really challenge AT&T/Verizon long term, which means that a merger to create three large players is better for consumers than a series of events that ultimately ends with two major players - and that seems like a fairly realistic outcome in this space.