I'm not a big fan of predicting the distant future. 1001 different things would happen in the next 15 years, and it's impossible to consider every factor.
That being said, the most probable outcome is that useless alts die, but many alts with good teams and good fundamentals will flourish and become very strong till then.
Nope it's not need 15 years to become popular the alternative coins, in fact there are some alternative coins that have a good future now and have a great performance. It's stable compare to the others. And also, there are many investor. I think it only need a 5-6 months before alternative coin become known