They can do whatever they want when it comes to 3rd contracts and beyond, I'm just going by what they typically do. An early extension just accomplishes what they typically do, push money forward. That's what they do with all of their players, push it forward when the need arises.
If the projections of next year's cap can be believed, around 225M - 230M, Dallas will only be around 5M over the cap, and that's with Dak's 50M cap hit. That's why I said I think they are preparing to absorb that cap hit. Because they can. What else do they spend money on? They spend on cheap free agents, their drafted players, and any extensions. They'll have the money for all of that with the cap space they are saving and rolling over. Remember, any extensions don't affect the team by that much the year of the extension. So even if they extend Diggs and Lamb, the only extra money going on the cap is 1/5 the signing bonus.