The fact that some (bad) programmers already blindly copy SO code does not detract from the original argument that Copilot is dangerous because it effectively copy-pastes SO code blindly.

It presents a suggestion, which you are free to reject. What's "blind" about it?

The fact that it doesn't come with context. I just fail to see the usefulness of the suggestion, if the quality can't be trusted. Either:

a) I'm familiar enough with the code/topic that I'm able to judge whether the suggestion is good, or

b) I'm not able to judge, and need to consult other sources.

In case a, I could have just written the code myself. In case b, I still need to read up, so the potential gain is that Copilot gave me a hint as to what I should search for. But even that hint can't be trusted - there was another comment where Copilot suggested to use MySQLi in PHP, which has been the wrong choice for a long time.

So if the suggestions need scrutinization, the target group I see is people who need to write a lot of code in a familiar area, and are limited by the speed at which they type. This is arguably a very small set of people, and is unlikely to be the main user group of this tool.

When I visit SO, I generally read all the answers and can pretty immediately judge "I think this approach is good" or "I don't think this approach is good" just by reading the code. I can't see why these suggestions would be any different. And in the perhaps more common case where I already know exactly what I want to do it can still save time to have the plug-in autocomplete it (I make heavy use of tab completion today after all).

The mysqli extension in PHP is fine to use and has advantages over PDO if you only ever plan to interface with MySQL. Now, if it was recommending the mysql extension, that would be bad as that's been deprecated and replaced by mysqli for a very long time.