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.