I also say that term of not keeping all eggs into one basket but in bitcoin, this is the opposite way. If you want to minimize your losses but to maximize profit, putting to bitcoin-only can be your choice. I have invested in other altcoins because it's my choice to diversify.
Bitcoin is a gem and if you have invested only into it, you have the best gem of them all. But as time passes by, you want to explore into altcoins and that's the start of knowing others of it through the researching.
yes, because investing in bitcoin could take a long time, unless the market conditions are good, it will easily increase.
investing in altcoin has different variants, because there are so many of them, at first it may be difficult to determine which currencies are worth trading.