Capacity could be improved even more, without fuelling the obesity epidemic and inconveniencing those of us who prefer to keep moving . All they have to do is make walking compulsory on both sides.
I think the point is enough people prefer to stand over walking than any dedicated walking space is likely wasted.
http://content.tfl.gov.uk/standard-tube-map.pdf
Until recently disabled access wasn't really considered. The Docklands Light Railway which started in the late 1980s, and the late 90s Jubilee line extension are the only fully accessible bits on the network.
Stations with suitable access are specifically marked on a lot of the maps for exactly this reason.
Looking at the 3D diagram of the station [1] it seems the original lift shafts didn't go to platform level anyway, and are now used as emergency stairs and a ventilation shaft.
Even if it's possible to add lifts, and even if the expense isn't astronomical, it would still only make sense if it could be done for many stations on the line — but most of them are this old, and handling far more passengers than ever anticipated.
[1] http://www.ianvisits.co.uk/blog/2015/07/12/3d-maps-of-every-...