Actually I would say, before thinking of going this route of investing or buying old coins, you will need to understand the type of old coin you want and your speculation about it , thus I am in agreement with your first paragraph. Crypto most of the times moves with trend, and this is key to the growth of most coins we see today, and funny enough only newer coins tend to tap into most the trend at that point in time, hence why it looks like old coins like the one mentioned have been forgotten.
Therefore the reason why I started with, understanding the old coin you want to buy is to know if the team of that coin are a flexible kind, who can try their best to incorporate what is happening in the crypto space at that point, if the team are then it's worth buying.
BNB is a coin whose team are always ready to try new things, hence the reason it is still relevant and bigger than most old coins. Every team needs to understand this, add their unique features and see their token regaining its worth overtime.
In this case I would think that there is a good chance old coins could take advantage of the new technologies as well, depending on the coin. Of course there are horrible coins that went lower and lower over time, but at the same time we had better and better improvements on some of the old coins as well.
Look at BNB for example, with emergence of DeFi world, they applied it right away and had BSC which allowed them to take advantage of all the profits from it, making BNB go from under 30 dollars to nearly 700 dollars one point and still 10x more than what it used to be. It is not a new coin at all and still managed to go up in the end.
There were also a lot of coins that were brand new and yet failed to garner any attention at all. All of which shows us that it is not about being new or old, it is about being able to adapt to whatever is the big deal of that period.