There is something you are getting confused over this, cryptocurrency can been known as those coin that is beneath bitcoin. Therefore, it was through bitcoin that gives raise for the formation of other related currencies such ETH, BNB, Dogecoin and many more.
One thing you must note is that, you can't just imposed someone to invest in bitcoin but rather you can tell them the usefulness of bitcoin and also to the entire cryptocurrencies.
Totally correct, bitcoin is a done deal for many investors, this simply means that, to them, bitcoin is no longer the ideal coin to invest in because it is already too expensive for them to afford the quantity that they feel they can make the kind of profit they want from.
I meet people with the above kind of mindset every day, when dealing with them, you cant just tell them to buy bitcoin and they would agree with you, you will have to explain in details to them why or how buying an already expensive coin is still profitable or going to be profitable for them..
Most of this people, after explaining to them, you will discover that they will prefer to buy some cheap altcoin they feel will bring them good profit, but because of your explanation, they will decide to buy their altcoin of choice with half of the money they intended buying the altcoin with, and then, use the other half to invest in bitcoin.