Vitalik Buterin believes voluntary sharing is no longer sufficient and that copyleft provides the “hard power” needed to enforce openness by requiring reciprocal sharing of code

Quotes that support claims

He said voluntary sharing is no longer enough and must be accompanied by the “hard power” of giving access to some code only to those who open up theirs.

Hence, the argument for open source cannot just rely on "please"; it must also be accompanied by the "hard power" of giving access to some code only to those who open up theirs.