Yes, Bitcoin was proven and tested when it comes to long-term matters. The question is, is Bitcoin the only proven and tested long-term investment in this field? what about the Ethereum, Bnb, Ltc, Doge, and others that are top-listed in the market are they proven and tested? Isn't that they also gave a lot of profit to al holders who believed in these altcoins?
That's your own take on this but I think alot of people who have been greatly affected by altcoins syndrome will certainly disagree with you on this because although the above mentioned coins have been around for some time but the fact still remains that these altcoins are nothing comparable to Bitcoin in terms of market trust and reliability. Let's take one of the so called popular coins that was a hit a few years back as an example
LUNA the coin had a big stand and alot of skeptical person like yourself who actually believe this stability actually invested their money on this coin and the after effect and toll was shown as all these coins belong to a central entity and can be controlled and manipulated which gives them the lower hand when it comes to their stand in reliability in the crypto space.