Peter Brandt as actually gotten this wrong, it is true that some altcoins are shitty and will not even exist anymore before the five years time he's saying, but more it can not be upto 99% of the altcoins. Coins like Ethereum, Dash and Doge will keep existing for a very long time.
The problem in here is, there are a hundred of projects created daily. If we are counting it, approximately around 36.500 projects created yearly. That means he's mathematically not wrong about it because 100 projects can be considered as 1% of our total altcoins right now.
Yup. We might have already forgotten 50% of created altcoins as they are dead already. And it's no wonder in 5 years time with the emergence of new technology and coins, most of the current coins would be obsolete, go out of any exchanges or cease to operate. Some coins would have hype but zero development, people would soon realize about them and as no new investors come in, they'll crash to zero.