But if you are holding things like eth, ripple, neo, eos, ltc, monero and other cool things, then just keep doing so, because if you sell everything for btc, you help increase its dominance, which is not a good thing for altcoins' market.
Lol. How is a decreasing altcoin market cap dominance good or bad for anything? It makes no sense. It's just a statistical metric that has no influence on individual coins you buy or sell into. Why would you care about altcoins anyway? If you believe in their fundamentals then you wouldn't be afraid of a decreasing dominance in market cap, but you are afraid it seems.
If a coin has really something to offer it will manage to grow on its own, if not, it will remain stagnant for years and slowly be delisted from most exchanges. Don't focus on market cap dominance.