The whole point of ports is not to re-do a lot of hard code. Plus, the code is already portable so the work they have to put into is minimal. For instance, do you think it would be feasible to make an entire PS1 emulator from scratch for the 3DS?
These ports have another great advantage. When a project like blargSnes is closed (seems to at least I havent found the source) as soon as the creator stops pushing versions you are not going to get improvements, but even after all interest is lost on the 3DS, you could potentially merge changes from the trunks of libreretro, because being portable means that whatever (big part, really, not all) they program for a regular computer is going to work on the 3DS cores.
Also, when these came out we went from having blargSnes and mGBA to a load of systems.
I think these are a blessing really.