Altcoins have always on the whole followed the BTC price but with higher variance. So if you bought a basket of altcoins, I'd expect it to behave similarly to a leveraged BTC position; when BTC rises, altcoins rise more and the BTC dominance decreases, and when BTC falls, altcoins fall more and the BTC dominance increases.
Exactly.
In general altcoin investments due to their tie to Bitcoin could be seen as a cheap investment in Bitcoin (without actually investing in it) with a 2-3x leverage applied. But of course, it works both ways; big profits and big losses.
Bitcoin -5.30%
Ethereum -14.5%
XRP -13%
BCash -8.46% (will mostly follow Bitcoin closer than any other altcoin for obvious reasons)
EOS -12.3
People however shouldn't get used to Bitcoin's dominance above 50% because it was meaningless at 40% and is still as meaningless at 50%.