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.
altcoins will not grow slowly. Their movement is always associated with a fast increase of their prices. These are not stable assets and that is why they attract risky investors.