Before Mixpanel can even attempt to solve that though, they first need to tackle multi-channel attribution. Right now GA and Adobe are the big players who tackle it, and GA only offers dynamic attribution (ie. the holy grail of attribution) in GA Premium (which costs $150k/yr).
If you want a standalone attribution platform you're left with options like VisualIQ, Convertro, Adometry, etc., all of which have recently been acquired by big players in the space.
So yes, I'd love to see Mixpanel answer the "why." However the first step is to truly answer the "where" question and until they add those tools, their model is still fully last touch, and therefore dated.
Of course even GA doesn't use its attribution modeling capabilities outside of the multi-channel/attribution reports, so they still have a ways to go too. The big difference is, those tools are available today for free.
That said, congrats to Mixpanel. They made huge strides in the event tracking approach that GA has since gone on to borrow, and that is what any modern web analytics platform looks at these days because it simply makes more sense.