Usually everyone piles on to the platform and waits. This causes crowding. Now you only really need to be on the platform for when your train is due. It works ok at other places. Take a look at Charing Cross. You wait on the concourse until your train is posted and then you head to the platform. London bridge will now have the same mentality. Wait downstairs till your platform is called and then head up the escalator.
That might be fine if you enter downstairs but people coming in on the upper level (i.e. everyone that walks across London Bridge from the City) will not want to go all the way down in order to come back up again. Maybe the upper concourse will be larger when finished but it's still pretty cramped at present, especially when anything goes wrong.
Generally if trains are running on time people prefer to be on the platform early so that they can get a seat, they soon get to know which platform a train generally leaves from. Personally I don't like waiting on the concourse and being part of a crush when the platform is announced.
Possibly ONCE there are more trains and IF people are confident they will get a seat they MAY wait on the concourse but otherwise expecting them to do so is not planning for the real world.
It does seem clear that some platforms are too narrow in places.