Further, I think if the issue came up, many of those who advocate landlord's freedom not to mitigate would still see a duty on the part of the landlord to permit the tenant to mitigate through the tenant's own efforts. If, for instance, the tenant proposed a substitute tenant at the same rent, the landlord would have an implied duty to be reasonable in accepting it. The landlord would not have to accept if the original tenant was receiving additional compensation in the deal, since then the transfer would not be in mitigation. And the landlord would not have to lower the rent, either, and ought to be able to demand that the new tenant be obligated and able to pay the full rent. I can collect the payments, because I sold the land.
Leasehold law works just fine.
There is no reason to tinker with it. The parties work things out in most cases as reasonable business people can do. The landlord really does want the property back and put to productive use. The tenant really does recognize that the landlord has had to address issues that the landlord had thought were resolved for the term of the lease, and does, or ought to, acknowledge that the tenant should pay a price for this. In fact, there are plenty of reasons why it shouldn't.