The alternative to using S3 is to do it themselves?
Also, do you really specifically want S3? The value of S3 is working at scale but very few but amazon needs that scale. So many other solutions work just as well.
S3 in regard of vendor lock is a bad take since it's probably the easiest service on AWS to migrate from with dozens of fully compatible solutions existing outside.
Even EC2 (AWS's VMs) require more work to migrate from.
S3’s lock-in is financial rather than technical, especially if you’re using Glacier for backups that need to be moved. It gets really expensive to pay for egress on all that data.
Even EC2 (AWS's VMs) require more work to migrate from.