Semenya already knocked off almost a second and has to knock off a lot more than that still. If you look at the 200s in 400s, Semenya cannot or could not get out as fast as the 400m runners, so that explosive start, from weight lifting, is needed before we see low 22, but I am sure we will see sub 23 shortly if there are competitions. 49.62 tells me that a sub 22 would eventually be possible.
Olympic 400m champion Ohuruogu could only manage 22.85. No chance