Part of the reason for the change was likely to allow better optimizations of the strings. The new strings are basically equivalent to arrays and might be optimized as such; even without that they certainly don't need an offset nor length field anymore since the underlying array can provide that.

If you conditionally used one implementation or the other you'd lose the optimization opportunity. You could alternatively make string non-final; but then your method resolution is more expensive and that would really be a pretty big change in any case.

I think java made the right choice here since its trivial to wrap string in another structure whenever you need a fast substring, it's just that that custom string won't be passable to lots of other API's which makes it a potentially nasty surprise for existing codebases.

In that case, it would be difficult for the programmer to reason about the memory usage of their use of substring().

This change lets programmers not worry about the implementation of substring() and memory leaks in return for linear runtime complexity.