Daniel Plainview is the personification of capitalism. His stubborn obsession for profit consume and turn him into an oil-seeking devil willing to give up everything(including his adopted son) for the promise of wealth. Much like Heisenberg from Breaking Bad, Daniel is an antihero in the sense that his actions and personality lack traditional heroic values. Some may see him as the movie’s antagonist but I believe the true villain is Eli Sunday.
Daniel is forthcoming about his intentions of finding oil whilst Eli uses the cover of the Third Revelation church(religion) as a tool to manipulate the minds of the people. Eli is a “false prophet” and secretly chases after Daniel for oil money. The “milkshake scene” is the conclusion to a longstanding battle between capitalism and religion in this film. Daniel has the upper advantage in that he has gained and lost everything he truly loved (including H.W) and nothing is worth fighting for anymore. Eli, on the other hand, is a desperate snake in need of quick cash. Daniel lays out the cards flat on the table and tells Eli that capitalism and religion had nothing to do with the oil digging and it was the man who would seize the opportunity the quickest that would reap its benefits.
The milkshake mentioned in the scene that both men possess is a metaphor for the opportunity to amass wealth(in this case, the oil fields) and the straw that Daniel seems to be the sole owner of could refer to the effort one must make before he/she can gain that wealth. Eli’s milkshake did bring all the boys to the yard but it was Daniel who ended up drinking it all.