The issues was raised with TfL a couple of years before the ELL was extended. The response was that they were aware that there would be crowding using the single escalator (and lifts that go between platforms via surface level ticket office), but that there were no plans to improve the interchange at the station. If I remember correctly they also felt that because the trains were only four carriages there was a limit to the number of people who would find themselves in a queue, and that the queue would clear well before the next train would arrive.

I think they actually prefer the bottleneck to occur before the escalators as this avoids large numbers of people suddenly arriving on a platform and trying to jump through the same train doors at the same time.