this is a pretty cool (as well as thought-provoking) idea....
so basically what you're trying to do is keep the same %gain as your sell trigger, but instead of selling all, it will only sell enough altcoins to maintain 0% btc gain/loss.
meaning that over time, your btc balance should stay the same while accumulating altcoins
this might not be a bad idea for a couple of different scenarios.
1. For Investment purposes. because it encourages diversification. anyone who uses the market for investment purposes will tell you diversify, diversify, diversify.
2. When you believe that altcoins will eventually overtake bitcoin. this should work in any situation where an altcoin becomes advantageous to use over bitcoin...even if that coin has a limited use. because in this scenario, even in a longterm bitcoin bull market, many of the rising altcoins will experience more value growth than bitcoin...meaning your bags have the potential to get heavier and heavier just sitting there.
that's all i could think of off the top of my head lol, but certainly there are other potential advantages to this strategy.
part of the problem with implementing something like this is that there is a distinction between daytrading/swing-trading and longterm investment. this idea sounds like it could work for a longterm investment strategy. in other words, you could use such a bot to manage your own altcoin index fund where you would place a longterm investment and leave it to grow...and over time your portfolio will expand, accumulating a diversified altcoin profit while your initial principle investment (i.e. your bitcoin balance) stays the same.
i'm scratching my head over this one... i would consider purchasing a bot like this even if it ran through a totally different program...