Bitcoin has a market cap of 70 billion. This is very low when compared with the stock market or the gold market. There are many groups that can buy the total number of Bitcoin in circulation with a price of $4000. They can easily manipulate the market both sides up or down. Bitcoin 70% supply is stored in a very less number of wallets say 5%. Don't you think Bitcoin can easily manipulate and below $1000 is still possible?
In a way you are right the market cap of bitcoin is very low and there are many institutions and individuals that could try to manipulate its price, but if they tried to buy as many coins as you state that will make the price of bitcoin to skyrocket because many of the coins are not up for sale and are being held by long term investors so as they keep buying coins the price goes up and it becomes more difficult for them to keep buying coins, and even if they could get control of enough coins to manipulate the market will they be interested in crashing the price when they paid so much more for each one of their coins?