Steve Jobs famously said, "If you do the right things on the top line, the bottom line will follow.”
Paraphrased: if you do things with the explicit goal to optimize revenue, it harms your business success. If you do things that optimize user experience and delight customers, it will provide more value long-term.
Voice assistants are in that latter camp, I believe. (And I think of this quote constantly as Tim Cook crams more ads into the ecosystem)
What ads did Cook cram in the ecosystem? The ones in app store?
Edit: IMO Apple is under-investing in Siri for that role.