Unfortunately, even if the decoder will be implemented, most HDCD recordings are not going to sound any better.
The available dynamic range has never been a problem with Red book audio. Only the peak extension feature would make HDCD encoding justified. In theory the peak extension feature could be used for providing two different masterings, a more compressed version for portables, car audio, etc and a version with full dynamic range for high fidelity listening sessions. However it is rarely used at all and even when it is used its effect seems to be very mild at best.
When the HDCD plugin for foobar2000 was introduced I tested over 2000 losslesly ripped audio CDs and found about 25 HDCD encoded releases. Only 4 or 5 of them had peak extension enabled and the extension seemed be to about 2-3 dB at most. After applying replay gain (i.e. matching the overall playback volume levels) it was almost impossible to hear a difference between the standard and "HDCD with peak extension" decoded versions.
To make things even worse, some of these found "HDCD" releases were compilations that contained standard tracks and HDCD tracks on the same disc without any visible HDCD labeling. Apparently the producers of these compilations were not aware that some of the original recordings were HDCD encoded, did not understand what HDCD encoding means, or just did not bother to fix the issue.