Why would you fully invest in bitcoin? I think it will not be suitable because bitcoin is a sound currency. It will do better in the future. I have everything in it to take a lot of opportunities from here and will do so in the future, but for this, I am, yes, I will end up entirely investing in it. I don't want to. I will invest as much as possible for convenience. I support gradual investment. You can do that too if you wish.
Sometimes people cannot afford to buy Bitcoin therefore they use an indirect method of investment in which they first buy low value altcoins and when it grows then they take out the profit and invest it in Bitcoin. Everyone is not wealthy therefore they utilize different techniques to become able to invest in Bitcoin and I think one can think better for himself because his financial status is in front of him while others cannot predict the money he has.
Investment of the full amount in bitcoin is a good idea and will be risk free but one thing is that we cannot take out profit after a short time from bitcoin therefore little investment is also important in altcoins which can give you quick profit and you can take it out when needed.