Loaning money to invest in a speculative and volatile assets is a bad idea in general. Yes, I'm bullish on bitcoin just as everyone else here is, but let's be honest: bitcoin isn't guaranteed to go up in the future, even in the long term. Borrowing money and putting it on bitcoin could put you into deep deep shit if bitcoin for some unforeseen reason drops in price in the mid-long term.
You are 100 right, however, a one size fits all answer may not be the option.
Like for example, many people recommend newbies to DAC (dollar average Cost) their BTC purchases.
I like this solution as it allows people to buy consistently, with money coming from your main income surplus into crypto.
So, if they had a budget of $100 usd a week, it would take them 52 weeks to buy $5.2k worth of coins with "surplus" money that they are cool to part with.
But what if, they see a great opportunity (a dump below $8k, before a massive rally and new ATH) and want to sink-in the remaining of their yearly $5.2k fiat ?
Maybe, a quick loan for $3k is a solution to buy the dip, and then they use the $100 allocated money to reimburse the loan.
Potentially making a better ROI than the $100 weekly buy of crypto.
So, taking a loan to buy crypto: no
Adapting ones buying strategy when an opportunity arises (without sinking more funds than the yearly allocation) : yes, if the risk-reward if favorable.