Alt coin are doing great most especially when it has potential and a good eco system to support it, the future crypto millionaires are majorly going to come from altcoins investors because a good alt coin has the potential to give you 100% turn over within a short period of time.
Altcoins even have the tendency of given more than 100%, I dont know of the altcoins we have as of now, but the altcoins that I know which I held some before the last bull run of 2017/2018 had their value even shoot up as high as 1500% at some poin.
So, altcoins are what you can invest with little capital and become a millionaire through it like you said, but we still have to invest in very good altcoins that have the chance of increasing that way with good products, otherwise, we will end up wasting our time with these shitcoins believing they will rise.