Altcoins do not complement Bitcoin because of the many altcoins today, on average they just follow the market flow influenced by Bitcoin. You can see how the condition of most altcoins when Bitcoin starts to rise and also when Bitcoin starts to correct its price in the market where they will also experience the same thing as Bitcoin itself. I will never blame big investors for this because they are also humans who always want profit with the amount of capital they currently have. So for small investors, they must also be wise in making steps in every condition.
Alts following bitcoin is true and people should just invest accordingly, but that's momentum based, not price. So that means, if bitcoin goes up 5%, then it means most coins will go up, of course there could be a few unique cases where a few of them drops, but that is rare, 99% of them goes up with it. The difference is that, just because bitcoin went up 5%, doesn't mean all coins will go up 5%, some will go up 1%, some will go up 10%, it is going to be different.
Also, there are rare cases when the market is stable or just moving small, when a coin ends up dropping or increasing like crazy, like a huge amount, that is just something big happened on that coin and not something in general. So, I am not really considering how this could change anytime soon but that will be the case I am sure of it.