there are many choices of altcoin that are good for long-term investments other than bitcoin, i personally have several altcoins that i still hold like ethereum, eos, xrp, xlm and bnb. in my opinion they have great potential in the future, if you are interested you can research again and try to invest with them.
It is not right to say that, because altcoins depends on bitcoin. If bitcoin can stay strong, altcoins will survive and grow. If bitcoin won't be able to do this, altcoins might collapse. In months to come till the end of this year, altcoins are good choices for investors, because we have not yet seen altcoin season. Nevertheless, bitcoin will still be a good option to invest, and hold till weeks closely to its halving time, next year.