"...Should the Saturn Astra have been a Cadillac instead?..."
Nope.
Should have remained an Opel Astra.
And Saturn should be killed off.

My thoughts exactly.
Astra isn't a sales success because it's too expensive for a car with such a powertrain when compared to Mazda3 and Rabbit.
If Astra was priced to compete with Versa it should've been fine. Add to this the fact that you cannot haggle on price when you can do that easily with any other car in this discussion. In order to be able to do that, GM should've built the Astra in North America.
Upgrading the powertrain closer to Mazda3 and Rabbit level wasn't possible for GM since the 2.0 Turbo does not come with an Auto in Europe. Spending money to create the right combination of powertrains for North America on a car that was approaching its EOL and it wasn't going to be a big seller in NA anyway wasn't a viable solution.
The result: a patch solution for GM, just to close a line-up gap for short-term.
Astra’s lack of success shouldn't prompt the conclusion that bringing Euro models here wouldn't work. It will work for sure if done right and hopefully Ford will do it.