For altcoins that have achieved ATH in 2017, I think it will never return, this market has dropped a lot and if it increases again, I think only altcoins appearing in 2019 will increase.
we do not know the future, indeed altcoin currently looks very difficult to grow. but look at its development from a few months ago, altcoin continues to grow even though its growth is still very small. so i think in the future, slowly but surely altcoin will reach ATH. might exceed ATH a few years ago.
The current obstacles are the reason for the low prices, the development and lower investor confidence don't allow the projects to reach the good points in the future. The previous ATH is x6 times more than prices of some altcoins and the market recovery phase can take more than 5 years considering current processes in the crypto industry. ATH prices are far away from this point and there is nothing left to do in this situation, unfortunately. 5 years is more than the required time for reaching the old ATH prices in my opinion.