You are assuming that as soon as a new coin is listed in a major exchange it is going to go up in price but the number of coins that do that is very limited.
Most coins as soon as are listed in an exchange go down in price immediately as investors begin to try to find buyers for their coins and then they realize there is no demand for them, when they realize this they begin to panic and as a result of this the price of their coins begins to go down even more.
That means you are buying bad coins, so there is no demand after the main exchange list. Actually the value of the coin falls there are several factors that affect it, such as:
There is no demand, investors sell at any price.
Too many bonuses
The most important thing for me to sell altcoin is more than the price you buy. that's the principle I use!
Of course not, I am not wasting my time investing in coins with no demand, I only care about bitcoin and some altcoins like ethereum.
And about the principle you are talking about that sounds easy in theory, you just need to buy for a low price and then sell for a higher price, but that is what every trader since the beginning of the markets has desired to do, but that is not enough, you need a lot more than that to get to your goals and unfortunately many of the tools you could use to help you with that like TA are useless with new coins.