Many people doing this for safe and secure investment. But if you want to multiple your capital then you must investment in altcoins. Cause bitcoin always has large marketcap and it need a huge investor to skyrocket from this current price, when altcoins can easily skyrocket within night. So you can't force anyone to buy bitcoin only, its personal choice. Well i suggest you to keep your bitcoin in your wallet cause if anyhow exchange get scam or problem happened with your account (kyc, violence issue) then It'll be difficult to safe your investment.
You are absolutely right that Bitcoin is a great option for investment but here your capital is definitely safe but the rate of return is also very low. First of all, you will need a good amount of capital to invest in Bitcoin. On the other hand, there are good and strong projects in the market where you can invest with less capital, and from there you can get a chance to earn a good profit. Which can be many times higher than Bitcoin. So I think one should definitely invest in Bitcoin but never ignore the altcoins in the market, because there are coins in altcoin that have the potential to give you a huge profit. Therefore, one should invest in altcoin along with investment in Bitcoin, but always choose the top and good coins of the market.