This is how many (most) jobs work. There is a bank of "Paid Time Off" (PTO) which can be used flexibly for sickness or vacation. The term bank is used purposefully to describe it, since it can be paid out as pay at separation. Many employers also limit the total amount of PTO an employee can accumulate. This is done to control potential liabilities from the bank.
While I don't call for the elimination of sick/PTO/etc, I also recognize that they are essentially irrational, similar to employer sponsored health care as an artifact of tax privileged "fringe benefits" that come with working in a large org. Funds paid to a worker for not working come from funds withheld from the worker when working. Sick/PTO benefit just makes a complicated system on top of the work/pay transaction.