Taking a loan for any investment always a bad idea, there's no guarantee your investment always increase and cover up the interest loan based on your math.
No, it's not always a bad idea. Taking a mortgage at 1 or 2% interest and more in an inflationary environment like the current one when you could pay the house in cash is a very good idea, for example, but I agree that in most cases it is usually not a good idea.
using your mortgage analogy
if i had $500k to buy a house outright. or take a $500k mortgage.. id buy the house outright.
first of all. even at 1-2% interest means at the end of the day that $500k turns out to be more then $500k you eventualy pay in.. secondly. you then end up needing to pay for insurance on the house to protect the banks ownership of the house when you are paying them.. yep they do not insure it for you for free. meaning you have to pay even more a month than just the loan.
then you have to maintain the house and are limited in what you can do with it, because the bank own it and want to protect their investment. EG its why banks offer re-mortgages but request you upgrade the kitchen/bathroom to modern standard to increase the house value(for them)
(pretty standard terms as part of 'equity release' contracts)
..
that said.. ethereum is soon going to 'detonate' into PoS.. meaning instead of being over $1k to mine 1eth. it will become $2-$15 to mine eth. and so ethereum miners will sell down the market, making profit. all the way down to say $20/eth
so if you are planning on having a 2 year loan where you can lock in eth value at todays value, even if ethereum is going to be worth 100-1000x less in 6 months. great do it.
.. or more simply just use your eth to buy a bitcoin miner out-right. and just start mining for btc.
as for mining. just make sure you live in a area with reasonable electricity costs. where you can get ROI quickly