Do you need all these solid facts and evidence to prove a point to your friends? You know what you believe and have already told your friends what you know about bitcoin. If your friends are really into crypto, they'd know which coin is the leading coin in the crypto world, especially this period that bitcoin's price has hit 100k+ and now around 93k. Which other coin comes close?
Your friends know what they saw in altcoins, allow them to do their things if they won't listen to you. If everything fails, it's their decision and not yours. Maybe they would learn more about bitcoin when they make mistakes with their investment choice. Some people learn better this way .
Like I really find it unnecessary to engage in such argument about which coin is the leading force in the market. I mean it is even visible for blind to see and I don't think it's worth it explaining to people who are not ready to be convinced. Moreover, since it's not partnership investment but everyone is responsible for their actions, it's not necessary bothering yourself trying to convince them. I think the friends are moved by the rapid growth and instant profit offered by altcoins and not looking at the long term reliability.
Op should have referred them to Luna for example and tell them how it crumble people financially in a single night. Many of altcoins do suffer the same thing and end up leaving their investors with nothing. It's normal to see market downturn but it's abnormal to remain stagnant over a year without sign of recovery. My simple advice for him is to allow the friends decide on whatever they choose to believe.
The beauty of the crypto space is in the fact that you are free to roam anywhere you like.
Believe in anything you deem worthy.
And be responsible for the actions and investments you do yourself.
Nobody would force you to do something, even if it's Bitcoin - and if a person doesn't see the benefit in the BTC, he didn't learn well.