For a B737, the most likely Navigation Accuracy reduction is a GPS loss. If this occurs then the ANP will begin to increase slowly (about 0.01 every 10 seconds) more than enough time to extract the jet from a terrain rich environment.
Dual GPS failure is mandatory missed approach if still IMC.
There is plenty of time before ANP exceeds RNP with just the loss of GPS and only navigating on IRS so it's no big deal!