Generally, taking a loan to start up a business or invest is not a good idea because of the added pressure it comes with. It's more better to use a loan money to finance an existing business and the interest rate must be very low, so that while paying it back in instalment, the business or investment wouldn't have to feel much effect from it.
But as of the case of Bitcoin investment, it's a bad idea because Bitcoin is an asset that should be held in a very long time, so thinking of paying back the loan with the money invested in Bitcoin is one of the worst thing a Bitcoin investor should do, because he might be forced to sell at a loss or prematurely just to pay back when the loan debt is due, so taking of loan with the purpose of investing it in Bitcoin is one thing we should all avoid, because it's not proper.
Actually its not a bad idea since how could you fund your business if you didn't take a loan to make it happen? Its just you making all of things work according to what you like to happen. If you scared enough to do this then could you get a result for having that feeling? For sure not. In Bitcoin I think its the same if you could pay back the lender why not? You are not trading it, but rather you are investing it for long term so with that set up I think everything is fine.
As long as you know what you do is good and you are capable enough to pay your loans then there's nothing wrong with it. The time it became a wrong decision to take, if you know for yourself that you cannot pay your loans but you still decide to get it and used since you will would really get a big problem with that especially by the time that the lender will ask the money what you lend to him/her.
Well I am having a hard time relating a loan to discretionary income, Bitcoin is meant to be bought with your discretionary income and here you are taking a loan to purchase bitcoin, it sounds counterproductive. The only constraint that would make you have my backing in taking a loan is when for any reason, your periodic earnings that fuels your accumulation were delayed and you, knowing that it would still come and not willing to miss your periodic purchases might well decide to collect a loan and service it with a view to pay back immediately it arrives. That way, it replaces your incoming discretionary income.
I don't encourage taking a loan for the purpose of purchasing of bitcoin, wether your salary is being delayed for your periodic purchase, you don't need to take a loan of any kind. Since your bitcoin purchase is for a long time and your accumulation phase is still long, even if you miss one month without accumulating, due to the fact that your periodic income is being delayed, it won't cause a problem for you. You can continue your accumulation once your payment arrives. You don't need to put yourself under pressure by going to borrow money. Secondly you don't know how long the money is going to be delayed, even if they are eventually going to pay you in the end.
You should avoid borrowing money to invest in bitcoin. You never can tell how long your employer will delay the payment. You will be in pressure if the money is delayed beyond your projected time frame. So to be on the safer side, don't invest at that period your money is being delayed. Only start investing again, when your periodic payment starts flowing again. It will give you more rest of mind.
You are right, taking a loan to purchase Bitcoin is wrong no matter the kind of strategy you are using you don't have to take loan for Bitcoin investment, however as a Bitcoin investor it those not mean you can't take loan if you have an emergency that has eaten up your backup funds, as long as you have a source of income or a means you can pay back the money is better than dipping hands inside your Bitcoin investment, if you receive $100 or $300 weekly as your salary and you have an emergency that has already eaten up your backup funds and you still need more $50 to solve that emergency you can ask for loan and pay back when you receive your salary.
What is wrong is going to take loan in other to invest in Bitcoin, Bitcoin is not an investment that you can go into and then make profit quick, some people take loan to invest in real estate and they get more than the money they borrowed in a short time, Bitcoin is different because Bitcoin is highly volatile in nature one can't really be certain and because of it's volatility taking loan to invest in Bitcoin may put you at risk especially when your hope in paying back with the profit you will get from it.