you should understand that most top coins that are currently in the top charts in regards to price and market capitalization weren't there few years back excluding bitcoin therefore the chance of them still been there in other years to come aren't certain. I can tell you for a fact that coins like Chainlink won't be in the too tens just as I speculated coins like EOS that was among that list last year won't make it this year.
While all you said made sense to this point highlighted above, you don't Invest or advice people to invest just because project are in the top whatever. We have scams project there too. Few years back, project like bitconnect that was a clear ponzi scheme scam was among the top 10 that means you probably would be suggesting people buy them and if they have did, today they would had been in losses as many Investors found themselves after they exit scam.
I both agree and disagree with your point about top coins. I agree that not all that top 10 was there and some of them will go down while new ones will come up and top 10 will constantly change I agree with that. Blockchain technology is something marvelous and we keep creating something better and better with it, and with more improvements in the future we are going to see better coins as well.
However I also disagree that coins like ETH could ever come down, or if we consider USDT as a "coin" that would probably be still high. So, I both agree and disagree because I agree some will go down while I disagree because some will stay up most probably. I mean look at ETH for example, it became second biggest and it doesn't look like it is going to stop anytime soon with how they are improving to stay relevant.