The Lambda functions will just be running your business logic code. Only specific part Lambda provides is the glue. Which is easy to replace with any other cloud or self hosted alternative.
But still, when I first started in AWS we had lift and shift'ed most of our platform onto AWS and when it came to adding new features I first looked at Lambda instead of building out on the existing platform. The speed at which I could setup the feature and start measuring and iterating on it was so much better compared to doing it on our existing platform. Even though that was pretty ironed out and fully automated.