I guess a rare alt will give you the best ROI. Bitcoin is currently in the $6,000 range and I don't think it's going to be going 100x like some low marketcap ALT might.
I'm not really sure what is the point of calculating good ROIs when at the end of the day we're still going to stay with an investment that guarantees that your money will not go to waste. With bitcoin being down right now, I think the more probably movement that will happen is for it to go upward. Alts are good investments and some people are putting money to undervalued altcoins in hopes that it will have its value back (or maybe even the ICO price) but it's undervalued for a reason.
It also depends on how active you are in trading. Some are risking their money to alts but at the end of the day, all of their money are returned back into bitcoin (called day trading). Some are able to sleep on it and just leave a sell order. The point is that if you're going to do altcoins, then you can't be just inactive because you might wake up and the coin doesn't have value anymore since altcoins have the tendency to die. Bitcoin is more for long term. It's something that you can leave and come back to after a couple of months and not have to worry much about it.
I completely understand your point, and I agree that Bitcoin is the safest bet, and that alts are very risky. I just think it's good to put a small amount into different alts, just for that potential of going 100x. Not shit alts, but once that might have potential. At least for people like me, which can be life changing. If one is rich, then I say stick to Bitcoin and other top coins.