Maybe I misunderstood your question, but potential is what something can deliver or reach in the future and not how much users it has today. The potential for crypto currencies if of course very huge, because people are tired of the fiat money and banking system as we lose the value of our money every year by inflation

I think yes
First of all altcoins have some future and many investor invest in the altcoins because they earn some profit of it, Actually ill earn a lot in altcoins because some of them have some potential to increase of there prices. So for now i'm still buying altcoins and holding them.

Yes you should some research about the ICO you want and if they have a working product that could be legit also you look for websites like ICO bench it will help you.