Search and Rescue Teams do most of their mission reporting in USNG (the National standard according to FEMA) or UTM and without this option embedded out of the box, they will not adopt Survey123 for their mission data collection.
Currently the Mountain Rescue Association are using the GeoForm to report their mission location - sometimes all they have is a report of where the person was rescued or last known to be and it will typically be written in UTM, USNG, or ddm coordinates in their incident reports. Then they have to summarize the incident and enter in a database - luckily the GeoForm makes this easy.
They would like to use Survey123 Web for the conditional logic so they can speed up data entry by using "rules" and for the built-in reporting / analysis features. This would save them a lot of time and energy. But they cannot do this without a way to plot incidents using UTM / USNG or at least degrees decimal minutes. They do not have a developer or GIS Specialist and are a volunteer, non-profit organisation. This is the case for most SAR organisations across the world.
There are probably many other Public Safety organisations looking for this capability as well.
Here is how it is implemented in the GeoForm - this works really well (although it is lacking degree decimal minutes).
Here are their standards for geo-referencing - notice USNG and Latitude / Longitude DD-MM.mm.
While decimal degrees may be easier for a developer to implement or a GIS person to geocode - the end user needs options based on what they are mandated to use. Their job is to save lives, not convert coordinate formats
Ideally there would be more options across ALL of ArcGIS Online Apps and Web Map Viewer - but Survey123 would be a good start.