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 
Well, this thread is old and it is too late to sell your bags that already lost 95% - 99% of its value. Your $1,000 is already just $50. There is not much to lose from $50 to $2.50 at this point. I firmly believed altcoins won't die because innovation and greed always come back.
You are right, this thread is very old one where people are in confusion state whether to hold the altcoin they are holding, whether if a person is holding a potential coin which is listed in top 100 in CMC will have a huge chance to increase their prices. There is a huge chance for these coins once the market starts recovering.