Some altcoins are still useful even beyond 2020, If the use case was still intact at that time i'm sure altcoins will be remain valuable. The existence of the cryptocurrency industry will determined the demand of altcoins including bitcoin in 2020 and beyond. My opinion was very simple until the cryptocurrency industry has a value the altcoins demand will be remain as it is.
200 coins in coin market may be in my opinion still worth in 2020, although there are some coins that are displaced it is natural,
there are many issues in this world, but I am not sure about the next 10-20 years maybe altcoin may not be able to survive
On the market there are still some top altcoins that still have the potential and opportunities to grow again in the near future. Perhaps judging the current altcoin is a difficult thing for investors. Altcoins are gradually being dumped out of the market because the dominance of Btc is so large that it is not self-developing but also depends too much. I hope the future is better for altcoin.