Who is ultimately accountable for post-launch product adoption?

I think it should be a committee of people, including product, marketing, sales, customer success, professional services, and other teams if applicable. They can own different aspects of the journey from pipeline generation, to win rates and revenue targets, to product usage and customer satisfaction scores.

It's a combination of PMM, Product, and your Customer Success teams. Product Marketing should continue to evangelize and advocate for adoption of the product through sales enablement, content creation, web landing pages, analyst influence. A combination of PMM and Product should promote adoption and usage through in-product notifications and guidance, making it seamless for customers to learn how to use the product. And, your Customer Success teams should have the relationships with their accounts to encourage usage, which in turn encourages renewals.