I view network egress as the primary lock-in mechanism for all of AWS.

If you want to migrate to another provider, network egress costs mean that you'll spend multiples of your normal monthly operating costs to do so. That stifles competition.

But well before that point, most services in AWS have already been built around avoiding network egress wherever possible. You're always going to prefer AWS APIs and services, even if they aren't the best for your use case, because services outside AWS have a network egress tariff placed on them. So you aren't always buying best-of-breed, you're buying the best of what AWS chooses to offer (or the selection of vendors that choose to build in AWS to remain competitive).

And if that isn't good enough, your only other option is to migrate out (and pay those egress costs!).