The problem arises now when employers pay the lowest wages possible. It must be morally indefensible for an employer who pays the staff who make their profits for them at a level that means they need state support to live. If the employer can genuinely not afford to pay more, and takes a similar level of income from the business as their employees, then maybe that business is not viable. But if the employer pays managers 20 times more than the shop floor staff and pays dividends in excess of their wage bill, then state support is actually supporting the managers, owners and shareholders, at least as much as the staff. To me that is unacceptable.
Actually is very defensible. You are assuming that all employees or potential employees are capable, physically and mentally of earning enough income for the company to cover the minimum wage.
Assuming a minimum wage of ~£10 per hour, this means that staff must generate more than that - to cover tax, NI, other benefits, time off, required equipment, office space, etc, etc, etc. If the staff member is generating £20 to £30 per hour, all good.
But what about those who can't? If the best they can do makes the employer less than minimum wage, they immediately become unemployable. As the minimum wage increases, more and more people fall into this trap.
Set a minimum wage of £15 and they need to be generating closer to £50, and so on.
We already have a large number of people who cannot be employed, and every increase in minimum wage increases this number.
Support for low paid staff means that the employee can generate a lower rate of return for the employer and still have a job.
Being employed greatly improves people's well-being, gives a sense of purpose and allows them to learn skills and progress.
A company I worked for years ago used to employ "unemployable" people at really low wages - it topped up the benefits a bit but was nowhere near enough to live on. The goal was to give people opportunities.