JPEG 9.1 is backwards compatible (9.1 decoders can read earlier spec jpegs) but not forward compatible (earlier decoders can't read 12-bit jpegs).
Agreed that JPEG 9.1 might not have the best possible compatibility with older JPEG; indeed I think it quite likely that the current JPEG will persist for a very long time, due to being an incumbent that is "good enough" for almost all users. One further point: oversampling and then down-sampling on display (like 24MP JPEG displayed on even 4K displays) also improves perceived DR over the "per pixel DR", so that is one way that even the current 8-bit JPEG can keep up with viewing needs, short of 100% pixel-peeping.
My point was only about the relative prospects of adoption for the various proposed replacement for current JPEG; the combination of some compatibility (including the potential for a phase-in of hardware support in newer devises) and the support of the the main standards body give JPEG 9.1 a lot more chance than the others.