Yes, they will wipe out the competition. The truth is all 3 networks are practically the same right now, they all have great coverage and good speeds. If a US telco (AT&T or Verizon) buys one of them you can bet "North American Roaming" will be included, and that's a big plus for business customers (you know, the ones actually driving up the ARPU, not RFD members). Add to that the volume discounts they get from Apple and Samsung, they can undercut the remaining two carriers on hardware (look at the US deals with "buy one get one free" iPhone and family plans).
That's if a foreign carrier enters by buying a network, we already saw that building one out from scratch (i.e. Freedom Mobile) doesn't make any financial sense.