It's a valid concern. Reasons why I continue to use it is because I like the product and I have trust in Automatic. They have a long history of supporting open source and it's far from their only product. They are making money via other products. At the same time, my life wouldn't stop if the product disappears tomorrow from the face of the earth. It would be a minor inconvenience.

my understanding is that Automattic pay for it because it doesn't cost them much and their income comes from other sources, mainly WordPress. That is a pretty stable stream of income, but you're right that they could be treating this as a vanity project unless it makes a difference to their bottom line.