Any “forever” or “unlimited” are forms of a Ponzi scheme.
Note: I not saying Wordpress is being malicious. I’m just saying that for the economics of a “forever” or “unlimited” to work, you need to find new money to offset the old costs - and a lot of really good companies have had to learn this lesson the hard way.
Companies also sell products and services below cost to build their brand and/or upsell the customers later.
I don't know WP's cost breakdown, but registering and hosting domains is relatively inexpensive and will likely get (relatively) cheaper in later decades. Meanwhile, they have a customer locked in for 100 years who will have a sunk cost incentive to purchase new features through WP rather than setting up a new domain somewhere else.