I would be amazed if more than 3 of the current top20 coins succeeded to keep their positions. The nature of crypto as small, emerging, and high risk market makes it hard for any project to maintain its advancement and that is due to many reasons, one of them is the competition between many entities that target the same domain, like the smart contract platforms, there are tens of projects in this domain and many are useless or just a clone of other projects. The other reason is the market cycle which enforce unprepared or unexperienced projects owners to shut down, therefore I expect many of the top20 will not be there when the next bull market starts.
If you mention the top 20, it is including BTC, ETH, USDT, USDC, BUSD, BNB, ADA, MATIC, DOT. Do you think these coins will be out of the top 20 in the next bullrun season? IMO, surely there will be more than 3 of those coins to survive in the top 20. I believe BTC, ETH, and stable coins stay at least, no reason for these coins to drop out. These coins have different concepts, they also don't compete each other. In addition, I am sure the developers/teams of the top 20 coins are experienced enough. They won't get many investors and won't get big market caps if they don't have a good experience. People won't trust unexperienced developers/teams.