I'm not saying supply isn't important, but it's not the only factor that determines a project's potential. Why do you say ETH is different when it has an unlimited supply? Because it was successful? It is also an altcoin, and in the past many bitcoin investors despised it, but now they cannot deny its success. I guess you are one of them, you only accept ETH when it succeeds, and you missed your chance with it.
As for alternative investing, it's more of a speculative market than an investment. So I wouldn't care too much about the supply or the use case... as long as it can make a profit, I will never ignore it.
There is a difference, in theory it is unlimited, but there is burning as well, which means that the amount of ETH could drop as well as go up, which means that it is not forever going up, it depends on the market. This is why we believe it's different.
Plus, it is definitely not the same thing as being second highest ranked coin and some other coin, of course being unlimited matters between them, when you are second ranked coin, that doesn't mean that you are going to end up with some devastating bad result suddenly, but when you are something lower, some other coin, then it could drop to very new lows and could result with something much worse. I believe that we can't really change that, it's going to just have to be the way it is.