Yeah, I'm sorry absolutely terrible idea from bottom to top.
Now letting my bias for a moment, there is a target population, as it's always the case with loans and it's in most cases people who have no other options, and people who strongly believe their coins will do an x3 or x5 after the next halving and don't really do the extra math when it comes to a 5 years loan. 14%? But I can put some coins in some staking scheme and get just from that 10-20 APR, enough to cover the interest. Taking such a loan when Bitcoin was $10000, great idea, right? Taking one when it was 60k...
There are takers, and unless Celsius and others are lying, there are thousands who would take such long-term loans. Of course, you have to take those numbers with a grain of salt, but seeing that they manage to lose $54 Million in one event and still stay afloat, seems the scheme is lucrative enough.
Oh, and one more thing, to us it does seem like a lot but for other countries, it seems like a small rate, some will realize too late that the interest is in $ and that is added on top of their currency devaluation, quite a few will end trapped because they've never had a second thought on this.