Smart people who had a bad experience wouldn't waste their time with an anonymous survey, they'd do a real complaint to corporate which is more likely to yield a coupon or freebie of some kind. Most people just go away mad and don't say anything anyway -- except on Twitter or Facebook, to their friends, not the corporation. So what they're likely to get is a bunch of relatively random opinions with little insight into the actual experience.
I had two horrendous experiences at the Sonic in our town when I first moved here, also with the Del Taco -- horribly slow slow slow service -- to the point that I actually boycotted each of those places for YEARS. I never filled out a survey or said anything to them, I just voted with my feet. I never went back to Sonic until I did my first mystery shop for them. Del Taco I tried again about once every couple of years, and recently their service has improved. (They're probably being shopped now.) Mystery shopping doesn't just tell them what's wrong, if done right it should keep the employees on their toes.
Not saying a company might not try switching to surveys, only that it wouldn't really prove anything because the people who enter would be doing it for the prize and have no reason to really think about their responses.
What's ironic is that the clients all think fast food customers care about friendly, cheerful customer service. They care about getting it fast, and not being sick afterward. I'll bet if you randomly surveyed 100 customers three minutes after they placed their order, they would not be able to tell you if the cashier made eye contact, smiled, greeted, or thanked. Nor would they care. What they care about is how long did they wait for food after getting in line, could they find a table, and was the food hot and tasty, the order accurate, and did they get sick later.
And, not being mystery shoppers, while they would CARE about how long it took, none would be able to accurately estimate what the timings were. They'd just know if they were late getting back to work or not.
I think our jobs are largely still secure. Even if some of them stopped Mystery Shopping I think they'll be back.
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I pray it does not occur that the last thing I did before I died was vacuum the house or eat broccoli.