Honestly, I don't see people getting unhappy at breaking private API functionality. Most grab a library when they need and never upgrade it. They only do so when there's a necessary security risk or some wiz-bang new feature they need.
The common example: programmer grabs a library to solve a problem; a few weeks later they realize it doesn't really solve the problem and requires modifications to the library to do some custom job; they make the modifications. That's normally the end of the story for several years. No point in upgrading the library unless it really solves a business problem.
That constitutes the vast majority of developers.
Of course there are other organizations that always want the latest and greatest version and are constantly upgrading. To them, I'd say sure, stick to the public methods, otherwise you're creating a big headache down the road. But I wouldn't require it of them.
The common example: programmer grabs a library to solve a problem; a few weeks later they realize it doesn't really solve the problem and requires modifications to the library to do some custom job; they make the modifications. That's normally the end of the story for several years. No point in upgrading the library unless it really solves a business problem.
That constitutes the vast majority of developers.
Of course there are other organizations that always want the latest and greatest version and are constantly upgrading. To them, I'd say sure, stick to the public methods, otherwise you're creating a big headache down the road. But I wouldn't require it of them.