[feanaro]

It is not a given that it will be milder, just estimated to be more likely once everything is taken into account.

As an example of why, the comment you are responding to mentioned the spike being functional (it is used by the virus for cell entry) and it is also targeted by our antibodies. So, for the virus to evade antibodies, it would have to change the spike enough so that antibodies don't detect it anymore. But, by changing it would likely lose some of its current efficacy.