With all due respect to your vast knowledge and experience, I don't agree with this.
I believe that most traders have issues with thinking too much about a trade, and becoming affected by the outcome of any one trade. If we set aside the need for a plan, which is of course the case for both day trading or long term/swing trading, then what we have left is execution. Simply, longer term trading doesn't allow for building this important skill. If you enter only 2 positions a week, and they are both losers, you will dwell on this, and it may very well affect your ability to put on any more trades the following week. Also, the losses might be bigger for longer term trades since your stops and targets will be bigger as well.
Lastly, if you're having too much time to think about the trade, you might start looking far too deep as for why this trade didn't work, and although there may be ways to increase a win rate by looking for other clues as for why certain trades shouldn't be taken, I'd say most traders have less than a 70% win rate, and hence being focused on squeezing out a slightly higher win rate might not actually be the best way to make more money. It might provide psychological comfort to have a high win rate, but this might not lead to more money, especially if it means less wins, along with less losses.
Correct me if I'm wrong Handle, but I do believe that you said you once has a string of 22 losses in a row, which you said was within the limits of your trading plan. If a retail guy had this happen to swing trades that are in the works for a few days each, 22 losses in a row might be 3 months of not having a single win, which would be hugely emotionally destructive.
So although day trading might not be easy, and tight stops might often kick you out of trades too soon, what it has going for it is instilling the proper trader's mindset. You put on a trade, you win some, you lose some, you move on to the next trade. Hopefully the losses are small, and hopefully the wins are a bit bigger, and hopefully after a series of trades you've squeezed out a tiny profit, perhaps break-even, or maybe a small loss, but if you're at least letting your trades hit stop or target on each one, you shouldn't lose too much. If you're always losing, then you've of course got a great system that you just fade. But it seems to me that most traders will start messing with trades, taking bigger stops, not taking full profits, etc., and this type of activity is I imagine even more pronounced the more time you have to think about a trade. So in many ways, I think day trading allows a trader to get used to working in the correct frame of mind.
