What Happens? You drop anchor and sit there until you get it sorted or the sun comes up.
Personally I run 3 seperate GPS sources - one that incorporates a radar which is invaluable for night time navigation IMO - especially when looking through a hard top screen. The second unit also incorporates a back up echo sounder for the stand alone Furuno colour unit and the third is a PC based system that is also full time hooked up to the engine for diagnostics which has got me out of trouble once - allowing an oil injection prime to clear an air bubble which was putting the engine into limp mode. Add to that the GPS in the mobile phone running the Navionics app for relief shading .............and hopefully I will never get lost LOL.
Having lived through a night time "GPS malfunction incident" in the early days of GPS that resulted in two breakers through the clears and a long, wet, cold night sitting in the open ocean not knowing exactly where we were and not game to move for fear of connecting with rocks in the area, I don't think it's stupid at all. There is something very comforting with having multiple independent displays all saying the same thing in my book.
AIS - is an option and isn't stupid if you spend a lot of time in areas with an abundance of shipping or trawlers. They still need to be looking at their screens to see you unless they have the alarms set on their units. You will need to complete a radio operators certificate of proficiency and apply for a MMSI number to go down that track though. Also remember that if you forget to turn it off when fishing, your "secret" spots.........won't be so secret to anyone in range.
Which unit - your call - but I wouldn't be looking at anything that utilizes data from the furuno or the existing Simrad if you are after true redundancy. If the furuno unit is being run as your main sounder you could look at a Furuno combo to allow easy interfacing for using the mark facility in the sounder to put a mark on the plotter but it will be an expensive option if you get radar to suit as well.