Of course as always many will be dragged down by heavy bitcoin pump/dump but the strongly driven coins with huge potentials will survive and maybe in the future will replace bitcoin itself on the top.
That's a big dream tho, you cannot replace the bitcoins or any altcoins out there they are all dependent on bitcoin and how come they would be top 1. Altcoins will survive some others will not but the sure thing is bitcoin and altcpins can grow more in the coming decades.
What? No, I think you should not have strong faith in the future of cryptocurrency too much like this. The coming decades mean next 20-30 years, cryptocurrency has not much chance to keep its trend still hot until a long time as you said. Only next 10 years for cryptocurrency is so hard to keep its trend on the world.