I don't think you can make a case for bitcoin destroying altcoins when a lot of them occupy completely different niche markets than BTC. BTC is only a currency coin, many altcoins aren't even competing against it, so how could they be destroyed/made irrelevant by it. Doesn't make any sense.
if you take into account Bitcoin or Bitcoin cash And also such as Bitcoin Gold, then any of these coins or similar other counterparts do not make sense at all. I think that it is much more complicated and more important in the world of crypto-currencies are those coins that provide real projects.