40 year old restaurants don't tank because of a Google listing; they tank because they've gone to shit, and from the reviews, that's exactly what happened. If you look back at the TripAdvisor history, they were doing Groupon and the like, which is in the case of an established restaurant is a sign of a restaurant that was already on the downswing. And a lot of those reviews were the same thing: poor quality food and poor service. If you've been around for 40 years, you should have a steady clientele that won't be looking for reviews in Google. You start pushing coupons when you've burned that established clientele by decreasing quality of experience.
I've been to restaurants like that; 40 years ago they were what was considered quality, but neither the menu nor the physical plant has been updated. The staff in the kitchen has gotten cheaper and cheaper, the skills are still 1970s level, etc. It's like eating in a ghost, and people don't come back.