- currency is very different from equities - while one could argue that a central bank deciding to peg it's currency to $ is a realistic scenario, it simply doesn't make sense for equities (an equity is basically a right for present and future earnings of a company - good luck 'perfectly' predicting that !)
- as you note, there would still be speculators, since there is always uncertainty. There are numerous examples of countries pegging their currencies and then when unexpected happens...
- traders / sales would still compete for order flow of the clients (and get the cut), so there would still be a game to be played albeit for smaller margins
* actions of any given platform doesn't meaningfully influence the market
* all have ability to forecast weather with 100% accuracy
* all have ability to perfectly predict human behavior
* all acquire the exact same news at the exact same rate
* all connect to the same exchanges with the same latency
* all have the same amount of capital to leverage trades
Simply put, there are incalculable ways for a trading platform to gain advantage over another, so I don't think this will be an issue in many lifetimes.
Yes - this... Buying or selling even 1 share in the markets adds information to this giant calculating engine. Anything you do will alter the course of the future.
I've never seen a backtested strategy that didn't look great on paper. The moment you drop it into the market - you can take a steep discount to your expectations.