It's a legitimate concern. Every line of code is a potential software bug. If you add 1,000 lines of code to accept, decode, and display ADS-B traffic (which, as a recovering software engineer, I would say is a very low estimate for the LOC necessary for those tasks), you add 1,000 opportunities for unexpected data, a noisy signal wire, or something else that was not completely tested before release to result in your backup instrument being useless when it's needed most.
Basically, software is very easy to write. You just take the expected inputs, the desired outputs, and draw a line between them. But the corner cases, where you get unexpected input, are where you find the difference between programming a web page for your pet hamster and programming an MRI machine or F-35 flight instrument.
It could be done, of course. But especially with the G5 being approved for use in type-certificated aircraft, it would probably be cost-prohibitive to completely test the new features.