Currently I own a lot of altcoin, I bought them at ico. My account is being downgraded several times. There are many people who say bitcoin will continue to decrease, so altcoin prices also decreased compared to USDT. So now, should I sell them and wait for the price to buy back? Give me advice 
depends... look at the technology and the team if they are constantly pushing the technology and more and more people are using it then in the long run it could be massive. too many people are out of a quick buck.
also remember crypto currencies still are not main stream. when they become main stream then value is bound to increase. especially of bitcoin