This reads to me as "let's make all state mutable and remove all indication as to which bits of the code are the interface, and which are implementation details". I like immutability and minimal interfaces, they make it easier to reason about what the code will do, freeing brain cycles to work on more interesting things than "but what if someone modifies foo between invocations of bar".

If you need access to a library's private or final fields, the library API is badly designed. Or, if you're that keen on wanting to change how the library works, fork it. That's kind of the point of open source.