I do love the way people think that Primeria came in at the cost of United and American. Both United and American cancelled their routes long before Primeria was even announced meaning it was impossible for Primaria to impact on United and American loads/yields. Primera was a opportunist/panicked response to United and American pulling their routes, they were NOT THE REASON United and American pulled their routes.
I also find this concern of "i hope one airlines doesn't push the other" to be counter intuitive.
Historically BHX has been defined as 1 airline serving 1 route (with the exception of holiday route). As such, 1 airline will only serve the existing market, They wont want to over supply that market and will manage that market to maximise yield - providing 80% capacity according to demand, the epitome of a monopoly. The airline benefits, pax pay higher prices, the market never really grows as pax willing to pay the price do, and others go elsewhere.
If BHX wants to grow however, we have to encourage competition in order to grow markets. The airlines often suffer in the first instance as prices become comitative, but what that does do is grow markets and grow demand over time. Yes their is a risk that the incumbent airlines yields decrease initially, but as a rule of thumb, airlines don't tend to want to hand their competition their profit just because their is completion on a route.
There is always a risk putting all eggs in 1 basket, as well as competiton. but if BHX is serious about growing and increasing market share, risks need to be taken.