That’s an incredibly narrow and unfair way to summarise the argument brought forward in regards of a possible legalisation. And I also believe that you’re not really understanding what whataboutery (horrible word) means.
You are ignoring possible positive effects from legalisation for consumer health. So what you are calling a flawed argument regarding the acceptance of a certain level of harm can be used to support the other side as well. There is a level of harm we accept due to certain drugs being illegal. The black market causes horrible harm upon societies, the lack of control regarding the substances causes harm and the societal stigma associated with illegal drug use also causes immense harm, as it makes it more unlikely to seek out help.
So legalisation might actually alleviate that level of harm.
In regards to whataboutery, the way I understand the word it doesn’t mean any comparison is always bad, wrong or somehow flawed. Reading your post that‘s the impression I get. I always took it to mean that people try to bring stuff into a discussion that has no relation to the topic. For example if I were to criticise Amorim for picking a wrong team and someone else replied „but what about climate change? That’s also bad.“ That would be whataboutery. But if a person were to compare his set up with that of his predecessor, that would be very valid. That’s the way I see it here. It’s just logical to look at the effects the legalisation of other drugs had, when we talk about the potential legalisation of cocaine, for example. It makes loads of sense to compare these things, as the experience with one makes it possible for us to form estimations about what might happen if we did the same with another.