I actually came up with this a while back. I think now, after going through this process, I can say this:
“You don’t do research to get a Ph.D., you get a Ph.D. to do research.”
Also a variant with “shouldn’t” would be appropriate ;).
Or in German:
“Man forscht nicht um zu promovieren, sondern man promoviert um zu forschen.”
Die Variante mit “sollte” ist auch passend :).