What if you are downstream from these changes, relying on a product that uses these APIs, and now you are negatively affected? You wouldn't be thinking that that their lack of planning or caution is a reasonable way of doing business.
Perhaps your due diligence should have caught this issue ahead of time, but it's far from unreasonable for people to apply research or experience when it's available.
You resolve the problem and carry on. Every business will have challenges like that. You can't mitigate them all ahead of time, at least not without missing a bunch of opportunities right now.
As a developer I find it hard to build things I know will break in the future. I find it hard to build things that might break in the future. Sometimes the opportunity in front of you makes it worthwhile regardless.
There is only diversified income streams (products). Not alternatives
No, it doesn’t necessarily make them careless. It could simply make them agile and adaptable. A bird can perch on a dying limb, grow, and fly away when it breaks off.
I don't think that's the whole story. "Don't build your core business around that API" ignores the opportunity that's there now. Your advice to build a business around something that can't be removed is sound if you care about the long term, which is sensible, but it ignores the short term immediate gain. You don't have to build a business that starts up and continues doing the same thing forever; for some businesses adapting to things like an API being removed is just another challenge to be met.
You're clearly quite cautious and like to plan far ahead. Other people don't do that. That doesn't make them wrong.