Nah, the Rhineborder was just an excuse for French expansionism. If France would ever have gotten the Rhineborder, it would soon find some other natural border to expand to (case in point, when the French actualy got the rhineborder in 1810, within months it annexed the rest of the Netherlands).
Although I admit that the French might actualy started to believe their own 17th century propaganda in the 19th century. That said, the French barely tried anything to actualy gain some parts of the rhineborder in the 19th century.
I disagree with you. The paradox of French expansionism in the 16th to 18th century was that it was defensive.
In the 16th century, the north-east frontier (and a hostile one since it was the Habsburg empire on the other side) was barely 200 kilometers of Paris. That was the main reason France pushed estawards and northeastwards.
And all strategists knew that the French border was still a defensive nightmare in France’s 18th century borders (which were almost the same as today’s).
This is why France needed a divided political map on its north-eastern frontier. And this was Richelieu’s and Mazarin’s masterwork to have secured for 2 centuries an ever more divided HRE through the treaties of Westphalia.
Now, I agree on the very limited character of possible french annexations in 1870. This was the era of nationalities. But it would have been a strategic triumph for France to separate all or part of Prussian Rhineland-Westphalia from Prussia and turn it into an independent German State, like Bavaria, Württemberg, Hessen, ... etc.