The second reason to buy the most liquid coins is that there are risks of scams. When you purchase Bitcoin you are sure that you will be able to sell it to somebody later. However, when you buy X coin, which is not as liquid and as popular as Bitcoin is, your risks are higher as everything depends in this case on the project, its team, goals, roadmap, background etc.
No, the successful trader is not me. I’ve gotten lucky a few times and I’m still refining and trying out strategies; on the other hand, I’m part of communities of people who trade on a daily basis to grow their portfolios, and while some of the results can be attributed to luck, a majority of it is based on fundamentals, good habits, and experience.