Essentially what you need is a crystal ball/seer/soothsayer.
Don't need one, I prefer to exchange opinions with people more experienced; just add their input/experience to my own.
That being said, if someone did suggest a coin to you and you went off blindly mining it, even at the expense of earning less than you would mining ETH. Would you begrudge them if you mined it for a week/month/year and then it ended up being worth nothing? Would it be their fault for suggesting it to you? Or, would it be your own fault, for not doing your own research in the first place and seeing what options are open to you, rather than taking someone else's advice.
Ofc, I wouldn't mine some coin just because someone suggested it, but I would put some time to do a small research on it. Better to have a starting point that to get on the 1500+ coins listed on CMC, right?
If mining ETH is the only profitable coin for you to mine at the moment, why would you mine something else at your own expense when you can just buy those other coins with your profits from mining ETH?
Here I would say for a cost of opportunity. choosing to win less (lose some) in the short term to gain bigger roi in medium term. As a comparison: Library Coin (LBC) has a potential of growth based on history higher than ETH - just by looking at the charts; so, wouldn't be smarter to mine LBC adn wait for the bullisgh market to return? This was my question...