What economic approach would actually work?

[Seamus Fermanagh]

France used its industry to build the obsolete Maginot Line. Their economy was wasted producing something that was useless.

It was expensive, and arguably other military preparation might have been better, but the Maginot line did, as intended, channel German effort Northward forcing them to face the low countries first and allow time for allied armies to position themselves to advantage.

The French screwed the pooch on their evaluation of the Ardennes, but the Maginot fulfilled its strategic purpose.