It's a reasonable timeframe, but not all codebases are actively maintained. In addition, it's concievable that there's some hidden custom library somewhere that crafts S3 URIs, making it near impossible to simply grep for a certain URI type in the codebase. So people may have to scour codebases they don't even maintain to look for random code which may craft an S3 URI in a certain way, then fork that project, fix the functionality, publish it, and use the fork. Then they may need to fork every other project that uses that original project, and do the same thing ad infinitum. If this is a private company, they have to do all that within some corp-wide globally available private repo, which either means (1) making this repo public on the internet, or (2) adding it to every security group they have that pulls code. It may even require adding direct connect or privatelink. So that means a long research project, followed by a project of fixing, testing, and releasing new code, followed by a project get network access to the custom repos and change software to use them.

So, surprisingly hard, but doable. And from the customer's perspective, a huge pain in the ass, just to save Amazon some pennies on bandwidth.