As long as the market don't go flat & stay that way for years than there is nothing to worry about. If you take a intra-day chart of Amazon or any other stock or commodity, intra-day ranges are no different than they were back in the 90's before the PHD's invaded. Trading costs have gone way,way, down & the retail trading platforms have gotten to be more like what the pros use. There are better instruments to trade now, like the emini's. Traders in this era are lucky. A trader can no longer compete against the HFTs & win the bid-ask spread with any consistency, not a big deal since there still a zillion ways to make profits if you put your work in.
Funny, I don't feel very lucky these days. What used to work for me 10-20 years ago stopped working today.
Basically, infinite liquidity from my perspective and account size. It is a different game managing billions of other people's money than your own accounts. Of course all trading strategies have a life span.
Partially true but since trading is computerized, they are not always building big positions (HFT funds). What they care about is the absolute edge potential in dollar terms. If something has the potential to return 100-200k per year, it's obviously inconsequential for their bottom line (effort/cost+risk).
Your walk forward testing should tell whether the model continues to be profitable when markets change.
The markets will change during walk forward because a time period is needed that includes bull, bear and flat markets.