Yep, BA sold Go to easy jet, which has totally destroyed BA short haul.
Better to own the genie and grow it at your pace, than sell it off, or gift the market to a competitor who won't grow it at your pace.
If Jetstar were not there, Virgin would be a lot bigger and still a LCC or tiger would be Jetstar. There would still be the same amount of frames in the market, Qantas would just own less of them. There would be more pressure on Qantas lowering its cost base to meet virgin as a low cost carrier. At least Jetstar gives you that buffer.