A great look at how one of the fundamental interactions between Sales and Customer Success teams can drift off course. I agree that the risk of failure increases when the key actors involved in constructing a deal becoming disengaged as soon as the ink dries on the contract. The challenges Greg outlines in maintaining your customer sponsor are also spot on.
One clarification I would make is that while higher level buy-in remains critical to your product’s long-term success for a variety of reasons, there are benefits to being “handed-down” to a Project Manager or implementation specialist. Your customer sponsor is likely to be focused on high-level goals and metrics (hence their motivation for shopping for a new SaaS product) and is being pulled in multiple directions at all times. This person will be an important resource for things like facilitating change management, securing resources for the integration, and allocating budget for price increases- but they are often not the best person to get in the weeds of the day to day of managing a product. Having a Project Manager whose goals align with your CSM’s implementation and onboarding processes is a huge benefit. To Greg’s point- just don’t let that Project Manager become the de facto owner of the relationship with your product.