There is not a huge fundamental difference between a global variable and a global interface. That doesn't mean that interfaces are bad, but it means you should minimize the number of globally accessible interfaces that you have, and where possible opt for local interfaces that are accessible only to the classes that are extending them.
In any case, peer dependencies are a heck of a lot more than just an interface. They're a shared implementation. That's way more dangerous.
You're looking at this from the perspective of "well, my peer dependency is the interface I've blessed." What I'm saying is that for any long-lived project you are inevitably going to get parts of your implementation, interface, and toolchain wrong. It is therefore in your best interest to optimize for small, encapsulated interfaces that will be easy to remove or change later.
Unless you're working on a trivial project, you likely do not know enough about your project to bless an interface. You almost definitely don't know enough about your project to guess in advance which interface future 3rd-party dependencies will be using. You absolutely don't know enough about your project to guess whether or not your dependencies can rely on an evergreen codebase rather than a static one that you test once and then leave unchanged for the entire component lifecycle.
It's OK if you think I'm wrong about that. I probably would not have agreed with this two years ago. And you could very well be right and in two years my opinion might flip again. All of this is just opinion me, I've gone out of my way to say that none of what I'm talking about is a universal rule - you are going to need to share at least some environment code with your dependencies.
But it's usually true. You don't need to join a cargo cart to understand that some ideas tend to be better than others on average. It's not that I don't understand my tools, it's that I understand that tools evolve.
In any case, peer dependencies are a heck of a lot more than just an interface. They're a shared implementation. That's way more dangerous.
You're looking at this from the perspective of "well, my peer dependency is the interface I've blessed." What I'm saying is that for any long-lived project you are inevitably going to get parts of your implementation, interface, and toolchain wrong. It is therefore in your best interest to optimize for small, encapsulated interfaces that will be easy to remove or change later.
Unless you're working on a trivial project, you likely do not know enough about your project to bless an interface. You almost definitely don't know enough about your project to guess in advance which interface future 3rd-party dependencies will be using. You absolutely don't know enough about your project to guess whether or not your dependencies can rely on an evergreen codebase rather than a static one that you test once and then leave unchanged for the entire component lifecycle.
It's OK if you think I'm wrong about that. I probably would not have agreed with this two years ago. And you could very well be right and in two years my opinion might flip again. All of this is just opinion me, I've gone out of my way to say that none of what I'm talking about is a universal rule - you are going to need to share at least some environment code with your dependencies.
But it's usually true. You don't need to join a cargo cart to understand that some ideas tend to be better than others on average. It's not that I don't understand my tools, it's that I understand that tools evolve.