As if "A" or "C" defined a person capacity. I know some straight A's that went directly for a repetitive and boring but well paid and stable job. Other stayed in academia and turned top scientists.

Academia is a very particular dynamic very difficult to find elsewhere, and some people dig it. You can watch some people finding the same dynamic at Google for example, where they are allowed and encouraged to fiddle around and keep publishing (e.g. the Attention paper, why would Google allow such publication?). Such dynamics are explored in Terence Kealy book "The economic laws of scientific research".

This varies widely between fields and institutions. Getting a PhD position nowadays in ML or computer vision is much harder. You need to already have publications when you apply and need to have experience specifically in the subfield, give a good talk, an interview, a good motivation letter / research statement, recommendation letters from good internships and multiple PIs you worked with, good grades, etc.

It can be different in other fields an in lower tier colleges.