It seems that every three to four years cycle new altcoins always takes a position in top 20 list on coinmarketcap, those who are leading the top 20 of 2017 are done for right now, only very few remains,
This is true. I've seen coins like EOS, Bitcoin Cash, IOTA, NEM and XEM at the top 20 at 2017-2018 but right now they are either in the top 50 or top 100.
now it seems this is what's going to happen in 2024-2025 too
I think it will really happen. We've seen many new coins at the top already like DOT, SOL, Terra (back when the scam didn't happen) and some coins went down.
I believe that new coins will emerge out of nowhere. New projects will come, and they might reach the top 10. It's possible, since we've seen it already.
now tell me why investing in a new project a bad idea? Cos I plan to look into new projects for the future instead, what do you think?
I don't see investing in a new project as a bad idea.
What other is saying is that, investing into new project has a higher risk of failing compare to those old projects with high market cap already. A new project means it lacks community and supporters. You can still invest into new projects and there is nothing wrong with it. Just be ready if something bad happens like that "new project" that you invested your money with turned out to be a scam or rug pull.