I agree. I have switched cloud environments at work in the past, and it required a lot of efforts. However we switched towards standard Kubernetes to have more flexibility in the future.

Also sometimes you don't want to minimise the cost but use the best services or reduce the risks. Let's say I go full on Azure, but after a year or two GCP offers a much better product for my use case. I will have to ignore it because it's too expensive to switch. Or if I go full on GCP/Firebase but suddenly the pricing model changes and it becomes overpriced. I will have to eat the cost for some time.

However, if I'm careful to avoid vendor lock-ins from the start, it will probably cost a bit more on average, but the maximum cost is much lower and I don't risk being stuck with a bad service.

In practice I will for example avoid to write code specific to the proprietary Azure Blob Storage API, but use an S3 compatible object storage instead. I will also rather have an abstraction layer than using AWS SQS or GCP pub/sub directly.