Southern are actually losing a platform permanently by the end of the development. So this is likely to not only continue throughout development but beyond this date. Queueing outside London Bridge will be a permanent fixture.
But they will be diverting some of the fast services (Caterham and Tattenham Corner) to use the Thameslink line, so they won't need so many terminating platforms after 2019.
At present there are two platforms out of action (as has been the case all year). After December platforms 9-15 should all be operational, giving Southern an extra platform compared to 2014 (one less than 2013 but the South London Loop was discontinued).
I'll continue to press for some of our weekend and off-peak services to use the Thameslink line or the Charing Cross line, so that there is some direct benefit for South Londoners.