If the ability to invest in Bitcoin is due to not enough money or little capital, then Ethereum can be an alternative in investing. Binance Coin (BNB) can also because it is currently cheaper than Ethereum so it is more likely to be bought.
You have a point however many people only concentrate on the short term, after all the only coin that we know that it is going to exist for the long term is bitcoin and bitcoin alone, ethereum and the binance coin could be there but there is no guarantee this is going to be the case, and in that case then it is better to just invest in bitcoin now that you can do so for a reasonable price instead of investing in bitcoin once it reaches 6 figures during the next years.
Investing in Bitcoin is always better than others even though we also do not say investment in other types of Crypto is not good. Most people who invest in other types of Crypto in the short term are people who make trading the main activity in seeking income in my opinion. Different from those who choose to invest in Bitcoin in the long term.