Automattic demanding 8% of WP Engine's revenues because they're not "giving back enough" to WordPress is a wanton violation of general open source ideals and the specifics of the GPL license. Automattic is completely out of line, and the potential damage to the open source world extends far beyond the WordPress. Don't let the drama or its characters distract you from that threat.
That's the deal. That's open source. I give you a gift of code, you accept the terms of the license. There cannot be a second set of shadow obligations that might suddenly apply, if you strike it rich using the software. Then the license is meaningless, the clarity all muddled, and certainty lost.