This is pretty easy to understand if you leave the flamming and insults out of it.
In the US, the FAA will allow you to treat the MDA as a DA if your training provides for VNAV approaches in lieu of the the dive and drive method of descent. Some here want argue that the D&D is better, or in some cases they just feel more comfortable with it. So be it. That not witstanding there volumes of data that show the VNAV or IAN approach is superior in every way to the D&D method. You can argue along with all the insults, but the data will still prove your wrong on this topic.
If on the other hand your not approved for VNAV along with using the MDA as a DA, then you will continue to use the MDA as your final descent altitude. I believe that the OpsSpec for this is B036, but don't bet the farm on that.
As others have noted the additional 50' was imposed on the operator so that they would not descnd below the MDA on a go around. In the case of Boeing the 50' is applied to Boeing aircraft from the 737 to the 787. No differences within the fleets.