I agree with you because if returns in investment in bitcoin is guaranteed,we will not be advised to use money we can afford to lose. Because if it weren't so we wouldn't be using a specific money to invest in bitcoin,a proof that investment returns in bitcoin isn't guaranteed so anyone investing in bitcoin outside of their discretionary income is gambling. Bitcoin is volatile, so investing only discretionary income would ensure investors don't freak out when they shouldn't or sell at loss when they shouldn't.
No doubt, it is advice able for everyone to invest in Bitcoin with their discreationary income and know that individual investors disreationary income differs depending on their source of income, why it is not adviceable for to go above our means to invest in Bitcoin is the pressure that may arise as cause of this.
I think you should know that there many uncertainties around Bitcoin as a digital asset, these uncertainties makes it impossible for anything to be promised or guaranteed, we all know that volatility is the big word, suppy and demand is another one, so all these mentioned play much role in determining the price of Bitcoin at anytime, and I ask, with all these factors involved, should anything be guaranteed, why long-term us much emphasized on is because, in a long-term, the price of Bitcoin is more likely to upsurge since volatility has a way of doing things.