Alts function similar to an index fund imo. A massive percentage of alt value is derived from bitcoin. If bitcoin's price increases, alts usually follow in its wake. The opposite is also often true. It may be possible to make an investment similar to a crypto currency index fund by buying bitcoin. Btc being one of the main driving forces determining overall increase/decrease in crypto market capitalization, allows targeting of average market movement to a decent degree.
If an index fund were implemented an interesting conversation might be had regarding how crypto index funds were divided. Bitcoin being a worldwide phenomena could there reasonably be dividing lines via geographic location? There might be a discussion on whether there's a point in dividing or categorizing crypto via some abstraction to make value easier to track. It could have long term potential in terms of legitimizing crypto and being a nice topic to talk about.
Yeah most altcoins crash hard when Bitcoin crashes. I think BTC is overall a much, much better investment in the long run because of multiple reasons.
Altcoins tend not to pump as much when Bitcoin does, but they do crash hard with BTC which means you get less upside and more downside. Also, they are harder to trade (less exchanges off them) and cash out for USD (almost impossible these days, only BTC can be cashed out for USD and that is even hard in itself).
Finally, there are often forks of the Bitcoin chain which essentially give us free money where we can just dump them whereas altcoins don't have forks and don't give free money. Based on these factors I think holding Bitcoin is hands down the number one option by a long shot.