Well, the issue with the S6 not fitting the multimedia doc isn't that the doc does not accommodate the S6 form factor...the issue is with the Micro-USB plug on the doc not fitting inside the S6 Micro-USB port.
That said, the Samsung Micro-USB cable that came with my S4 fits the S6 just fine. Micro-USB is supposed to be a standard plug.
Upon further investigation, I noticed the Micro-USB plug on the multi-media doc has small grooves on the back end that are not present on other Micro-USB plugs I've seen. This may be the culprit, and might be for the very reason you mentioned....just to get you to buy a new device each new phone iteration.
I have not seen any new Samsung multimedia docs offered yet