So you believe that MA's work because enough people to believe them that in combination they buy or sell sufficient to move price? So indirectly, MA's move price?
Are you really saying these things?
MA's are not a self fulfilling magic number prophecy type indicator like say a Fibonnaci number. The latter only really work if people believe them. But an MA just tells you the price has moved recently. Using pretty much any sensible indicator/filter will give you exactly the same answer. There is almost no difference in outcome using a 'magic number' MA like 200, and using say 211 or 189.
So MA's only 'work' or move prices in the sense that:
a) they tell you the price has gone up or down - which isn't a special property of MAs and certainly not a special property of MAs of a specific period
b) a bunch of people like following trends, enough to move the market
c) those people will trade in such a way to continue the trend. And they would do it regardless of whether they were using a 200 day MA, 211 day MA, todays price - last years price, the beta or ^2 of a price regression, or some more complex filter; or just drawing lines on a chart by hand.
Long term moving averages are used by Wall Street. Finance professionals may use them to justify investment decisions.
I'd agree that simple 200 day moving averages are used quite a bit, though not at the more sophisticated end of the street. Importantly though the more sophisticated indicators give you almost exactly the same answer as a simple MA (albeit they are preferable due to other properties, like stationarity and lower turnover) so from the outside you couldn't tell exactly what indicator someone was using to trade with.
GAT