it is interesting to consider the consequences of failure of any of the three instruments most commonly included in a dive console: 1) a compass, 2) a depth gauge, and 3) a SPG. That consideration should also extend to the 4th 'instrument' - a timing device. IOW, what do / should you do if a gauge fails?
For ME - and I cannot say that my choices are the best, for anyone other than ME:
1. A compass
I generally do NOT carry a back-up compass
If my compass fails, it is very unlikely that I will terminate the dive, UNLESS that dives necessarily involves compass navigation.
2. A depth gauge.
I generally carry one DC on recreational dives as my depth gauge, and two or more DCs on technical dives as my depth gauges.
I generally know to what (maximum) depth I am going to dive, be it on a solo recreational dive, a buddy recreational dive, or a technical dive, before I enter the water. I also know my time limits at whatever depth I plan to dive, based on my pre-dive planning. On a solo recreational dive, I will (probably) go ahead and terminate the dive as soon as practical (that may involve swimming to an exit point) if my depth gauge fails. On a buddy recreational dive, i will probably link up with my buddy, and use their depth gauge as my depth indicator. On a technical dive, I usually carry two DCs, for depth and time information. So, if one fails, I will terminate the dive and begin the planned ascent to the surface, using the functioning DC. If both DCs fail, so I have no depth or time information: a) if I have a buddy, we will use his/her depth gauge for deco stops; b) if I am solo and have no depth gauge (very unlikely), I will - yes - guesstimate.
3. A SPG
Failure of an SPG is pretty much an auto-call - I am going to the surface. How I get there is a function of whether it is a recreational or technical dive, whether I am solo or with a buddy. But, I generally DO NOT carry a back-up SPG for back gas. So, an SPG failure on a recreational or technical dive is a basis for a controlled ascent to the surface. For many years I carried a 'brass & glass' SPG, AND used wireless air integration. At present, I do not have wireless AI, so my SPG is IT.
The bottom line in this thread - failure of gas supply information is a reason for a controlled ascent to the surface.
4. Timing device
Time is one of those data points that many divers take for granted. I certainly did, early in my diving 'career'. GENERALLY, on a recreational dive, loss of time information is not a serious issue, for ME. My gas supply will determine my bottom time, and I have no problem using my buddy's timing device on a recreational dive. On a technical dive, time information is critical. If I lose my timing information on a buddy technical dive, I will begin a controlled ascent, according to plan, simply leaving the bottom depth early. I WILL use use buddy's timing device, within NDLs, and certainly for decompression stops. If I am conducting a solo technical dive, It is very unlikely that I will suffer a complete failure of time information. But, if I do, I will - yes - guesstimate.
Again, these are MY general practices, as best I can organize them for purposes of this post. I do not suggest that others should adopt them. But, they may be a useful data point for consideration.