But contract law went well beyond that long ago. The crux of the matter, therefore, has to be whether leases are so different from other contracts that mitigation should not apply. I can collect the payments, because I sold the land. I'm not convinced. And the court would second guess whether the seller had acted reasonably with respect to his unique situation.
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.