There is no "forcing". If there is sufficient demand for paying with cash, then someone will start a business that accepts cash and charges more, thus making more profit. More people will start such businesses, driving down the price until the price accurately affects the additional risk plus market demand.
If no such business exists, it is because the cost of the risk/hassle of dealing in cash is too high compared to the low market demand.
This is akin to saying that if there were sufficient demand for free speech rights, privacy, or the abolition of slavery, markets would provide for those things - and that their removal is simply an economic function.
Like, if enough people want handjobs from non-sex-trafficked sex workers, surely some massage parlors will open to cater to ethical customers, right?
Markets do not guarantee civil rights, and they shouldn't be relied upon to do so. Moreover, their failure to do so should not be construed as a proof that broad enforcement of those rights would not improve both individual businesses and the wider economy. Nor should the economic impact be the sole reason for society to choose whether those rights are worth defending.
By your logic, businesses can choose to refuse service on the basis of race because a certain race is poorer on average - and people from that race just have to hope they will be served by another business that comes along to hoover up their money.
We as a civilization have decided that we don't want the market or a collection of individual businesses to make those sorts of decisions. Protecting people from predatory discrimination is the role of civil society and government, and why we make businesses conform to certain standards, whether they like it or not.
That’s a lot of words built on top of a rather flimsy assertion that using cash is a civil rights issue, which I don’t think any court has actually ruled on. It seems like a right that people have invented from thin air.
The answer to people being unbanked is to make banking easier and less discriminatory. Not to force everybody to accept cash.
Classes covered under Title IX aren't necessarily covered by the Constitution, but our democracy has recognized that they need protection. Civil rights are rights outlined in law and reinforced by the courts. They're not pulled from thin air. If you believe that there is no right to spend your income without first depositing it into a third party for-profit bank for safekeeping, then make that case.
Okay, then let's not force businesses to make their buildings wheelchair-accessible either, because most customers can walk. If some customers can't shop there anymore then the market will take care of that problem too, right?
“The law, in its majestic equality, forbids rich and poor alike to sleep under bridges, to beg in the streets, and to steal their bread.” - Anatole France
If no such business exists, it is because the cost of the risk/hassle of dealing in cash is too high compared to the low market demand.