For skilled traders, the problem with "day trading" as a main focus isn't that it's impossible but that it's stupid. Longer holding periods offer dramatically better strategy metrics (as measured by e.g. profit factor or per-trade expectancy) at a small fraction of the overall effort, far less time in the market, far lower commission costs, and with no requirement to operate at 100% peak focus and performance for hours on end. Capacity constraints mean that you won't be able to deploy more than a fraction of your capital in day trading, unless you're just starting out - and if you are just starting out, you won't make money because day trading requires higher skill compared to swing/position trading.
This is why you don't see self-described "day traders" in Market Wizards and similar. Almost by definition, those at the top of the field (in trading or any other field) are the ones who make maximally efficient use of scarce resources, which in our case means capital and the trader's time/attention/focus. Day trading does neither.
EDIT: Not in the mood for a debate.
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