If you ask me I will say only buy Bitcoin but I see why latecomers are attracted elsewhere, there are other cryptocurrencies with unique features & potential for large growth. Diversification is key for some people, It is advisable to research & understand the potential of different cryptocurrencies considering factors such as technology, adoption & market trends. Each cryptocurrency offers different opportunities & risks so a balanced investment strategy that includes a mix of cryptocurrencies may be more beneficial for some in the long run.
Diversification is definitely a good thing but the problem in my opinion with that is that not all altcoins even those with promising technology and other stuff, you're still risking a lot of your money in diversifying in hopes that you can hit a lot of altcoins and get more profit when you can just buy bitcoin and hodl, it's more a better option that diverse number of crypto because with bitcoin, you're guaranteed to make a profit if you're patient unlike altcoins that are 90% likely not to go up after one peak in prices.
How ironic when diversifying your investment is exactly the same reason as what you have in your statement, "not every altcoin in the market is capable of pumping". However there those that are capable of multiplying your capital. None of these coins you are certain with or none of the investors knew what happens with a specific coin. Diversification isn't the problem, choosing the right coin is, that why we need to diversify to try not to miss anything that will pump in the future.