The price only goes down if there is more supply than demand. So more people that want to sell then those that want to buy. Like any market, as the price drops new investors will see it as an opportunity to enter a market and start buying, creating new demand. This stabilizes the price and finally will start increasing the price as more and more people feel confident to buy. This is a reoccurring phenomena you see on every stock / commodity and bitcoin market.

Holders of big amounts of bitcoin (or any other market asset) are usually careful by not flooding the market to such level it can create its downfall. On traditional markets this is even illegal in some cases. In crypto currency they might use this power to create sudden price drops, to be able to buy back cheaper. This would be conspiracy theory, as I'm not aware of real prove of such thing happening.

Well, many people forget that Bitcoin is an investment as anything else(like stocks, bonds, real estate etc).

If Bitcoin goes to zero, only the guys who invested everything in Bitcoin will be hit hard. If they diversify, and have lets say 10% of their capital in Bitcoin- it won't be such a big deal, as 10% isn't really that much :)