Yes the cap will go up, but what if some other running back gets a lot more money, then Zeke's value will go up, and your additional cap money would be eaten up by that possibly.
No, the Cowboys don't have to sign Dak, nobody is saying that. But he won't sign a 1 or 2 more year deal, it'll be at least 4, and even if you sign him for like $35 mil a year, in 3-4 years that will likely be a bargain, as long as he and/or the team improves.
If you think there's zero reason to sign Dak now, fine, but it's not a black and white decision...
At the least, if the Cowboys go away from Dak, then there's no guarantee the new guy will be any better. No guarantee he won't of course, again, nobody knows, you pays your money and takes your chance