We flew our RV-10 for many hours on a D10A and an EMS D10, then upgraded to the D180, then the Skyview, then the Skyview Touch. The whole time we had an Airspeed, altimeter and TruTak ADI as a backup. By far the cheapest way for you to get some redundancy is to replace your D120 with a D180 and get a backup battery for it. The number of failures like Springer experienced is very low, probably orders of magnitude lower than the number of vacuum system failures. The next simple upgrade would be to add an independent backup like a Garmin G5 or a GRT Mini. Even a Dynon D2 would be a good backup. Obviously, you could spend over $30,000 upgrading to a new system, and you would LOVE it, but only you can determine if it would be worth it. If you are planning to sell soon, then definitely do not do much upgrading. If not, then I would not consider resale value a big part of the decision (a part, just not a big part).