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.
However, you shouldn't make this a constant habit, when you see a shift in the arrival of your income, you can as well adjust your accumulation moments to suit it rather than being a regular customer to loans. Reason being that on the long run you would've lost a very good amount as interests paid on loans and if you could adjust your accumulation periods to suit your income arrival, you can utilize these interests paid to increase your aggressiveness in accumulating bitcoin or use it to build up a cashflow reserve to accumulate bitcoin for moments that there is a shift in funds arrival.