Many altcoins are based on scams, they appear only to make money for themselves. Many projects have seized on investors' greed for profit and raised funds for their own interests, rather than for the development of the project and the interests of investors.
That is what the scheme is and the old users like me know how such scams run under the convenient cover of anonymity that bitcoin and the internet has to provide them with. The ones who fall for these scams are mostly the newbies who seeing the price of bitcoin cannot afford to buy whole number of bitcoins and end up buying thousands of shitcoins that are worth the same but will surely drop to zero in a few months.
Point is that there will be successful altcoins that reach near to what bitcoin has been but these are rare and they will rarely be picked up unless you are getting lucky. Every team seems to be working hard but below the table you never can be sure. Hence keeping bitcoin as the max investment yields the safer portfolio.
Scams are getting a lot more sophisticated as well, there is a chance that we could end up with something that is a lot worse in the size of it but a lot better in the quality and still scam people. I have to say it's not an easy thing to handle and it would be a tough sell.
I just need to make sure that I get my hands away from those good looking new stuff because they are just a facade and nothing more. If you want to make money, stay clear out of them and focus on the old ones that has a future and could go up a lot with the bull run, that's how I make my money so far. I know it's going to take a while but it is a good way of making money for sure.