Leaving you parrt-way down an approach, with no accurate navigation information...
All the RAIM messages in the world won't help you find your way out of trouble, when you have no tracking aids available, nor the current ability to interpret them...
At the risk of being boring, that isn't Map Shift.
If you lose RAIM during a GPS approach (or GPS guidance), then you fly the missed. If you lack the guidance for the missed, you climb back above MSA and revert another aid.
It would be possible to sound unduly alarmist about GPS approaches. They work remarkably well.
2D