IMO the D check is, or should be priced into the lease price. One of the reasons a lessee pays a markup on leasing a plane vs. the actual calculated value of the plane is for the flexibility and the option of returning it at the end of the lease period (or at any time, upon paying termination fee).
Personally, I would not be surprised or annoyed if anyone leasing my planes at a markup returned it right before the D check. The lease fee covers the D check and then some. On the other hand, if I gave someone a deeply discounted lease (such as 30% off) as a favor to them, then I would expect them to do the maintenance also and not dump the plane right before D check.
brique, you make a good point that sometimes the lessor can be left holding the bag unexpectedly. For example, an airline that signs a long lease can BK.
However, what happened here was not a typical leasing situation, as the owning airline had flown the plane from the beginning of the game, and was planning to keep flying the plane a year later. He only put the plane up for a 1 year lease to make someone else do the D check for him, and he even requested a feature that would force the plane to be kept for a longer minimum period.
A years-long lease will have a mandatory lease period of several months before the plane can be returned. A one-year lease will only have a 1.2 month mandatory period. But this airline wanted to both cap the lease at 1 year, and have a feature locking the lessee in for 2+ months, just to handcuff the newbie or desperate lessee to the bomb.
Had someone leased this plane for a few months and returned it to the lessor, it would not have left the lessor unfairly holding the bag, because the lessor was flying this plane throughout its entire service life during the game (and planned to continue to do so), whereas the lessee was only allowed to get a few months' service out of the plane. So, while I agree with your general point about the roulette / musical chairs, this case (and this type of case) is not an example of that problem.