However I disagree with you, even though conditionally when you talk about investing what you can afford to loose. This notion contradicts your initial belief in cultivating an emotional strength to hold your bitcoin for long. It however tends to tilt in line with the Bitcoin Traders anthem, which is investing only what you can afford to loose.
You said it above in your post that one must invest with his discretionary income if he wants to be a long term bitcoin investor. I don't see how investing with the amount of money that you can afford to lose contradicts investing with your discretionary income. Don't you know that your discretionary income is an amount that you can afford to lose. If you don't invest with your discretionary income, it means that you are not investing but gambling.
You can do whatever that you like with your discretionary income, some gamble,trade,go clubbing, take care of their wants,and some drink with their discretionary income because it wouldn't affect their way of living. This is why it's wise for you to invest with your discretionary income into bitcoin and save your discretionary income from been a waste but increase the value as you use it to buy and build your bitcoin portfolio for 4-10 years and above.
if you keep accumulating a part of your discretionary income in Bitcoin regularly, it will help you risk-free and stress-free investment
Don't get it twisted, DCA does not make bitcoin a risk free investment. DCA helps you to build your bitcoin portfolio gradualy to a better sixe overtime based on your commitment which you might not be able to achieve such sixe of portfolio if you were only using the buy dip method or lump sum method.
Bitcoin investment is like every other investment with risk but the risk can be managed. This is the main reason why we are advised to invest with only the amount of money that we can afford to lose so that you don't use all your life saving to invest in bitcoin and hoping for a big profits, because what will you do if bitcoin price goes against you are didn't give you your desired profits. You will become fustrated and give up in life. That's why you have to create other streams of income as you are investing and prepare for the unexpected so that your bitcoin investment wouldn't be seen as your las hope in life even if you have reached your bitcoin accumulation target.