Actually, what was just implemented was changing long-inactive bug reports from Incomplete to Invalid, not setting them to Incomplete in the first place.
As for this particular request, I don't think it would be a good idea. I'm subscribed to dozens if not hundreds of valid bug reports that have been open for years. Having to actively keep them open every month would cause a lot of noise.
Actually, what was just implemented was changing long-inactive bug reports from Incomplete to Invalid, not setting them to Incomplete in the first place.
As for this particular request, I don't think it would be a good idea. I'm subscribed to dozens if not hundreds of valid bug reports that have been open for years. Having to actively keep them open every month would cause a lot of noise.