I would argue that most cloud native services existed in their standalone forms way before public clouds made their own versions. For example there are loads of message queue systems that are just as easy to incorporate and are cloud agnostic, some of them are FOSS. Sure you can reinvent the wheel but in most cases something like RabbitMQ will work OK depending on the use case. Having cloud vendor lock in is where cost catches up with you. Complexity is arbitrary since there are ways to make anything overcomplicated.