The only way BRS would build an extension to the runway would be if the major operator (i.e. easyJet) were to state that in order for it to expand further it needed a longer runway for destinations not currently served (such as SSH).
Given EZY seem to be making loadsadosh from the current network range, there would never be a sensible economic argument from either side, not now or the future, particularly as modern aircraft are ever-improving their power-to-weight potential.