Who is ultimately accountable for post-launch product adoption?

Totally depends on the organization. I've seen it owned by the product manager, the product marketer, growth lead, etc. I'm stating the obvious here, but at some level, every function in the organization has a very important role to play. While every goaled initiative needs an explicit owner, I strongly encourage you and your org to declare and report on the adoption goal at the company-level. It will help drive the right mindset.

My personal take:
1. Product marketing is best positioned to own and be accountable for usage and adoption of the product holistically. Product marketers have the best view of how it all fits together and the most direct influence over how the entire customer experience (website to product to comms) fits together.
2. Product managers should be accountable usage and adoption of the features / parts of the product they manage. After a launch, the PM should be using feedback (usage data + voice of customer) to continue improving the feature and the value it delivers over time.

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.