The Abandonment of the Gladius for the Spatha - Why?

[Thiudareiks Flavius]

I'm observing a discussion of this question elsewhere and find most of the arguments presented unconvincing. They include:

(i) The Late Roman Army was on the 'defensive' and so abandoned the more 'aggressive' gladius for the more 'defensive' spatha.

(ii) The Late Roman Army was heavily barbarised and so gradually adopted the spatha because it was preferred by 'barbarian' troops over the gladius.

Any better ideas than these? Both of them strike me as deeply flawed or just plain silly.