One of the COMs could fail.
Yet, there is no requirement for duplication of radios – unless required by some STC.
On the general approach:
If, if, if… There is a saying in Czech, which doesn’t rhyme in English, along the lines of “if there were no fish in the pond, they might be in your ar*se” 
This is quite a puritanical approach, because in a typical SEP you are much more likely to lose the alternator (which is a crappy unit from a 1960s Ford truck but selling for $1000 with basically worthless “traceability paperwork” to keep the regulators happy) and the battery will go flat fairly soon afterwards.
So a risk assessment which leads to two panel mounted units (when actually a handheld GPS of some sort would be the logical nav backup; a VOR receiver is farcical nowadays in Europe) in order for an STC to be approved, is largely an artificial exercise where all the old chums in the regulatory business support each other
But they can’t allow a handheld backup because handheld stuff is not TSOd/ETSOd so no bread on the table at home of the regulator.
I see this at work with the old scam called ISO9000. It means absolutely zero, zilch, nowt, nothing whatsoever regards the quality of the actual product.
I remember a CAA presentation where the CAA said (regarding GPS) that they asked the FAA if they ever did a calculation of the reliability of GPS and the FAA replied “no”. This is obviously BS in a number of ways. But this is how the regulatory process works.
