You said that "DCA should be done by excluding unnecessary expenses" although it sounds good, but all expenses are not irrelevant. It will not be right for a person to reduce his standard of living or spending on entertainment and social things and exclude them and invest in Bitcoin. After meeting all his family and personal needs, if there is any discretionary income, he will invest in Bitcoin from there according to his convenience.
Not only with DCA but also with all investment strategies as basically you must prepare money for your investment capital and to do this well, you must manage your finance well and minimize your spendings. The more savings you can have, the bigger investment budget you can have as partial saving fund for your investment.
If you work and use all money from salary for spendings, you can not have spare money for investment. There is an risky option like borrowing money and buying bitcoins but it is very dangerous with loan interest repayment calls, loan expiration and unpredictable volatile market.
Investment is only good, safe and profitable if you use your own money, and more important discretionary income for your investment.
This is a reasonable and practical opinion of DCA and long-term investment. One can make more savings by cutting on unwarranted costs but it must not be at the expense of reducing his/her standard of living or avoiding healthy social and personal activities. The only thing to invest in should be after attending to family needs, personal needs and saving of the emergency money. The safest source of DCA is discretionary income since it will keep the investor relaxed even when the market is volatile without worrying about the financial strain. Proper financial management is an automatic result to the enhancement of investment capabilities in the long run. Spending all the salary money on expenses is pointless as it does not leave a dime to invest, and taking a loan to purchase Bitcoin is a grave threat. The best way in which sustainable investing can be employed is through the use of personal spare funds.