You are missing the timeline factor here.

2016 - lets use EC2, its just VM, we can move off

2018 - I see you are hosting your own PostgreSQL in EC2, you can use our managed solution

2020 - you are already using 18 our services (note, at this point you might still be using non-vendor products, like VMs, managed DB, and so on), why not use our IAM instead of rolling out your own auth.

2024 - you are now deeply locked, lets add more lock-in, why don't you use this tool to optimize your costs (welcome DynamoDB)

At this point, no one would ever question next tool from salesman. Because engineers see that company doesnt have strategy to move to another cloud, why should they reject this new tool?

also consider the people who are involved, a lot of times after 2 years you have totally new people in your team, they won't have context and constraints you had in the past when deciding to buy "just VM", they see it as "we already use AWS"

2025 - You start using Amazon S3 with pre-signed URLs and serve those directly to your customers. Great, now your customers are also locked into AWS.

That. Also add the parts that engineers are terrified to their bones of moving elsewhere because they don't know how to use anything else and will act as extension of the salesman to make sure they don't need to learn anything.