The purpose of the security deposit is to protect a landlord from a tenant who has nothing to lose by damaging their property.

If a tenant has no money, there's nothing to collect or keep, and a landlord has to eat the cost of repairs themselves or suffer higher homeowners insurance payments.

In your examples, 1 or 2 are reasonable usage of a security deposit.

3 and 4 are abuses by landlords that must be prevented, but there are measures to do this.

In the state of massachusetts, for example, the law is set up to favor the tenants, allowing them to sue for treble (3 times) damages if a landlord tries to pull that stuff.

If I, as a landlord, keep a $1600 security deposit under your scenario #4, and the tenant sues, I could be out 4800. The tenant would trivially be able to get a "free" attorney who wants a cut of that sum. There are even guides on how to sue a landlord in the instance of abuses.

Your scenario would increase rents, as homeowners insurance would increase. With no security deposit to collect from (especially in the case of poor tenants) insurance would pay out more often.