You are right, we should not take loans for business and investments although many big businessmen also take loans with interest to run their businesses why they do it, it depends on a number of factors like to save themselves from taxes or they must have fixed assets.
Using leverage is always risky for either an individual or a company or a big institutional investor. There are success and failures but you can not think that if you accept risk, make loan, use high leverage, you will get success but not failure.
The key point is control risk, avoid loss as much as possible, as if you can not have good risk and capital management, with either small or big capital, you will have loss, not profit. With trader or investor, the worst thing is losing your initial capital that is like your blood. Without capital, a trader and an investor finish his job, and can retire to find another job to work for money.
I am still very bullish on BTC I did DCA in BTC last year and made around 3x from it till now but I own a small part of it. I still think $1 million for 1 BTC is achievable but I still won't take a loan to buy one.
DCA is proven as a very good strategy for investment in long term and with very good ROI of Bitcoin over years, DCA Bitcoin is an effective investment strategy.
https://dcabtc.com/https://casebitcoin.com/charts#roi_chart