Not especially, there will also be some "churning", but a third rwy represents a huge increase in available capacity and the "churning" is a relatively small part of this.
For U2 and other UK carriers to establish LHR operations is a natural progression. Existing and new carriers, both UK and overseas, will be able to add new destinations.
Some of these may be thin routes between LHR and smaller UK airports, and that will give many of them a lifeline.
All sorts of possibilities open up with extra rwy capacity.