Don't think your early stage startup will ever have that problem?
That's exactly what a lot of other people thought 5-10 years ago, and now they're stuck in S3 which means they need to use EC2 to manipulate that data which means they may as well put it all in AWS because egress fees will eat their lunch.
Also for that amount of data it'd be faster to transfer as a stationwagon full of backup tapes (https://aws.amazon.com/snowball).
That still means to exit AWS you have to add 2.5 months of opex to your budget, which will weigh against the perceived benefits of relocating the service out of AWS. That isn't an accident on Amazon's part -- there is a business case why ingress to S3 is free and egress from S3 is not free.
There are also a lot of problems around paying both the old and new provider during the move, so if you aren't very careful you could wind up going from a $2.1 million monthly opex to $9.2 million until the project wraps up. And what projects end on time?
Snowball may speed up your timeframe and reduce costs, but 100PB still costs $3 million to move and you still need to go back and sweep up the data from S3 once you've verified the data is complete.
People forget that when you talk about customers spending 100+ mil/year they get heavy discounts on many of the SKUs.
Egress fees on the order of 40-50% off list are possible in my experience.
Dx list with 100 PiB is 2 million. Add in discounts and you’re at much less than 1 mil total
Even the few that don't, you can use something like Minio.
I wouldn't consider S3 something that requires lock in.