The decline of physical mail is undeniable, but it's also La Poste that delivers the vast majority of the packages coming from e-sellers (and we all know that's just been increasing over and over). Also, the "high job security" you talk about does not exist in this company as pretty much nobody is hired as a "fonctionnaire" (a state worker) anymore, and it's been years now. The workers in La Poste suffer from a very high pressure from middle management who cuts jobs and readjust "tournées" (a mailman perimeter) incessantly, with the same "there's less work than before" excuse. The reality is that these people work more and more (significantly), work conditions are worsening drastically, and La Poste employees have one of the highest depression and suicide rates in the country (more than 40 last year declared in their farewell letter they committed suicide at least partly because of the job, I let you imagine how many more were not counted).
This new product (while I think it would be really great if workers had enough time and an appropriate training) is unfortunately just another way for the company - and it'd be fine if it were not, again, to the expense of the workers - to make more money. Implying the main reason is that workers just don't have enough to do anymore is just not fair.