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Given the pain that would be associated with rolling your own, why not make the case that a new method be added to the String interface that provides the old behavior? Legacy applications still need to change, but it would be a relatively straight-forward mechanical replacment.

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That requires storing an extra two fields per String (int length, offset;), which is costly. Users who need constant-time substrings can simply implement their own class `Subsequence extends CharSequence` with a constructor taking a CharSequence and two ints. Users who need to pass a substring to a foreign function which only accepts String do need to copy, but that's not a major enough use-case to justify upping the memory usage of most applications.

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