I think it's more than you realize because it was more than I realized. It only takes getting burned by a cloud provider once to get the execs to notice, then they talk about lock-in, and the good ones try to avoid it. Plus, remember people move companies and take their knowledge with them. It's the kind of thing that gets talked about in CIO circles [0], so it's more than a tiny minority.
In contrast to provider-native serverless solutions, cloud agnostic solutions carry a high price tag for the entire application life cycle in form of operations - those people petting your k8s. A common misconception regarding serverless applications is focusing on compute alone and in that context an agnostic solution may seem lucrative after traffic reaches certain threshold, justifying running a cluster 24/7. However many scalable application architecturs benefit from asynchronous processing and event-driven models which require reliable messaging infrastructure with considerable operational overhead. This is where serverless applications utilizing managed services shine, making it possible for small teams to deliver very impressive things by outsourcing the undifferentiated ops work to AWS.
On the other hand, if the compute layer is the only lock-in-inducing component in your architecture, a properly architected application is relatively easy to migrate to a serverful model. As a crude simplification, just replace the API Gateway with Express.
0. https://www.protocol.com/enterprise/target-cio-mike-mcnamara... (Disclaimer: I work for Target but not currently on cloud stuff, though did previously)