If we have purpose to increase amount of BTC we should trade that BTC with others altcoins, this way is risky so we should be carefull and make better prediction when time to buy and how many percentace of profit that we should be take it. We can see every day some altcoin become the biggest gainer. So trade BTC with others altcoin is way to increase BTC, and do not forget use stop loss to avoid big loss from altcoin crash.
People day trade for different reasons and some simply do not really look much into daytrading btc-fiat pair but would rather prefer to take advantage of the fluctuation with the altcoin market to increase their number of bitcoins. However, I understand what the OP is saying about day trading btc-fiat since the entire markets move accordingly with the way bitcoin moves, which is actually true, but that is in a downtrend, but we also have to understand that altcoins can move up faster sometimes to a great value compared to bitcoin.
I have seen a lot of coins that you can easily keep increasing your btc stash with without stress and for what it is worth, I have seen that when I used to day trade altcoins, I tend to see more potential in gaining much more than just daytrading bitcoin. The only difference here is that you still need to use bitcoin as an indicator most especially when it comes to it making a move downward or whatever condition it is that results to an unfavorable one for the alts, and that makes it a lot of additional work to do.