Ethereum is a great investment but I would advise you never to keep all your funds in one coin - you are much riskier.
Totally agree with you never need to keep all your money in one currency. it will never lead to good results. but if you distribute your funds to several promising currencies, then this can bring a lot of profit
Yes, diversification of investments is simply necessary in the current conditions in the crypto currency market. Thus, you will reduce risks and increase the chances of a good profit. And you need to leave part of the funds for the reserve, if the coins fall even lower, you can buy coins at an even lower price.
I have done that and it works great especially the part where I don't have to worry that much if the price of a certain coin dumps because I still have other coins with goid prices to depend to. Putting all money in one coin is a lot riskier than diversifying it. Ethereum is a good choice to invest to but still it will be profitable if a trader will look at at other coins who has the potential to grow.