Quote from TSGannGalt:
No one can.
The basic premises of any model or trade is "If the current condition sustains...
... the prices can move up to xxxxx.
... the trend will continue.
... the market will reverse.
... the system will make $xxxx.
... the system will return $xxxx.
etc.
etc.
The difference is how you define "condition" and the conditions change. It can be trend... it can be tendency... it can economical condition... it can patterns... it can be players.... order flow.... it can be anything about the market.
It's the "market" part of "Markets always change"
What you just said takes no more than an understanding of basic algebra to apply. I've met a lot of these so-called quants who understand very little of advanced mathematics.
Real mathematicians are not so simple minded to consider markets stationary or in any linear fashion. There's a lot of research with very promising potential in financial forecasting with concepts such as support vector regression, Kalman filter, and etc.
I've met a lot of other math majors in college who're simply incapable of thinking outside the box, and they end up hurting reputation of financial engineering. It's like any other industry, you have a few winners who stay low profile, while a whole lot of losers get publicity.