The problem is that most people earn less and hardly meet their expenses. With inflation rising every other day, it is really tough to spare extra income for middle averaged salary person.

I don't consider you as a middle class earner if you don't have spare cash from your income at the end of the month. You fall under the category of poor people. Those poor people who you said that can't invest into bitcoin because the don't have a discretionary income, some of them drink beer, gamble and smoke cigarettes. They can use that money to buy bitcoin and hodli while they work on improving their income.
Sometimes it can be unclear whether a person has discretionary income or not.
Of course, if a person is drinking alcohol, gambling and smoking, then those seem to be activities that are discretionary and they are not needed to sustain life.. so theoretically, that money could be used to buy bitcoin instead of using that money for those other non-essential purposes.
Many times people will have some discretionary income that they would be able to use to invest into bitcoin, yet the amount, such as $10 to $20 per week might not feel as if it is enough to make any major difference, including that if the amount is relatively small, it could take way more than 10 years before the amount invested might start to seem like a lot of money.
For sure, we cannot answer for others, since sometimes there can be psychological and social advantages in regards to spending some amount of money on consumption goods.. and sometimes it can be quite difficult for some poor people to cut their expenses and/or to increase their income, which may well be what is needed for them to be in a position to be able to invest into bitcoin for 4-10 years or longer.. so not only do they have to put the money into bitcoin, they have to keep earning enough money so that the are not tempted to tap into their investment in less than 4 years.. which can be quite difficult, especially if they investment into bitcoin goes into considerable profitable price territories.
Even though one starts small, the real advantage comes from habitual accumulation, transforming bitcoin saving into a routine financial behavior rather than a reactionary one.
A further possibility lies in income diversification: rather than only cutting expenses, one might look for creative micro-earning opportunities (freelancing, digital tasks, or peer-to-peer value exchange) that can directly fund bitcoin purchases. This method separates investment capital from daily living money, reducing the emotional pressure of spending whats meant for savings.
Another strategic angle is value alignment, using bitcoin as a motivator to think differently about personal finance. Over time, that mindset shift may encourage broader financial literacy, goal setting, and resilience building. Eventually, this combination of consistency, diversified earning, and long-term conviction can compound far beyond what the small numbers initially suggest.