Altcoin scam comes from coins that are mined before a project is launched, the coins are sold before listing the it on exchanges, the developers can leave the project on the day it supposed to be launched and take away all investor money. Some developers can just list the coin and sell all their coins which they have from the beginning, making investors to lose in the dumping process that can make the project become useless and having no future. Going for bitcoin is better to avoid scam.
Yes, Bitcoin is the safe haven in the crypto investment scheme but the altcoin scam happened in many ways not just only those you listed because the dev team not keeping his promises is also a scam which we can also list Vitalik among them when the ETH2.0 roadmap was released.
I know some ETH enthusiasts won't like my statement but it is the truth.