Then no progress would ever occur in may areas. For a new concept / idea / product to be introduced there always has to be a first somewhere @:-)
Substantial modelling, risk assessments and simulations will occur and a huge safety case will have to be completed and assessed. The safety margin distance between the end of the landing section and the start of the take-off section of one long runway will be substantial to cover every possible overrun scenario. I'm all for it - let the UK lead the world in this concept rather than us sit back and always think of a thousand reasons why something can't happen.
Great - let's do it at an airport in the UK which isn't the busiest and where an error could lead to tens of thousands being delayed.
Why not build that at Birmingham maybe ... or Manchester?