I'm sure you realize this, but it's important to know that it isn't a lease, it's an operating permit. MSG owns the land that it sits on and the building, so no lease is required. Just because the permit expires in 10 years doesn't mean that the city can then do with the land whatever they want. Hell, the permit could expire, the Rangers and Knicks could play in Hartford (gross) and the city would just have an empty building unused in Midtown.
They wouldn't be able to do anything about it unless the Dolans sold them the land or they went through an eminent domain proceeding. But, as I mentioned a few months back on the previous page, the eminent domain thing would involve the city paying market value on the land, which I think would be easy enough. The problem is that it would also involve finding an equivalent, suitable site for the Dolans, which would essentially double the price, or more, of just buying the land.
But, as you said, this probably goes until the permit is near expiration. At that point, the Rangers and Knicks will have recouped a lot of the cost of the renovations they just finished, making the ownership more amenable to trying a Garden relocation again.