The aim for both is to have something that behaves as closely as possible to the original. What differs between the DM1x/41 series and the DM42 is precisely how that is achieved on a technical level.
There is no copyright on the ROM of the HP Voyager and 41C series. It is therefore possible to write an emulator for the underlying Nut processor and run the original ROM within that emulator. This is precisely what the LPC1115 based SwissMicros machines do.
This changed for Free42 and, by extension, for the DM42. There *is* a prominent copyright notice in the ROM of the HP-42S. Using that ROM in another product, commercial or otherwise, is therefore a strict no-no and is one of the reasons, If I understand correctly, why Thomas Okken wrote Free42 as a completely new application simulating the operation of the HP-42S rather than an emulator using a ROM image.
What this means is, the suggested alterations to the way the DM15 works mean actually rewriting the HP-15C's ROM image, and without any formal documentation of it, eg. a full listing, that means reverse-engineering the whole thing. A larger company might well have the resources to do this if they were willing to commit to the expense (none have TTBOMK). SwissMicros is just Michael and David working flat out on the DM42. This isn't going to happen any time soon