1. Take a talented, valuable developer off whatever important task they're working on now.
2. Get them review whatever changes have supposedly been made between old version and current version.
3. Get them to read the existing code and think about places where it may break. (Often not obvious.)
4. Deal with doing the upgrade, which may often have dependencies that can break other unrelated things.
5. Write whatever code needed to make changes. (Often not obvious.)
6. Test on some sort of staging setup, which was probably written by some other person a while ago and may or may not have adequate coverage for this specific feature (since it was already working!!!).
7. Deploy and test in production.
8. Hope and pray nothing breaks.
All of that work (time and $$$) must be done to achieve ZERO benefit for your customer. High cost! Zero benefit! That makes developers (and founders) angry.
1 year deprecation is insanely short for public-facing infrastructure APIs. We should be talking about 10 years.